Jacqueline Nought
Role:
Jack acts as one of ME3's sources on where Cerberus bases are. She is an temporary squad-mate for breaking into the Cerberus – bio-engineering labs during the Shadow War campaign, which (provided she is alive) she locates. Jack can also discover the Cerberus synthetic-engineering and mercenary training facility. During the Shadow Broker's final assault on Cerberus's Cronos Station, if Jack is a renegade and Kai Leng kicks her in the cargo bay (during the first part of the mission – before fleeing), Jack, unless romanced, will go into a rage and pursue Leng against Shepard's orders. Her rampage will clear the way for Shepard and SB troops to take control for much of the station.
Recruitment:
Liara discovers that Jack was last located on the planet [x] in the Terminus systems where she had gone in search of a biotic gang. Liara suggests that Shepard talks to Aria T'loak for more information on Jack's whereabouts.
Aria explains that she hired Jack to help her take down Cerberus bases located somewhere in the systems surrounding Omega. Unfortunately, Jack's last transmission stated that she was going to check out either a pirate base or civilian compound, in order to gain more information and support on where the Cerberus bases were.
Shepard takes the Normandy to the planet, scans the terrestrial body, and discovers two transmission signals for places where Jack might be located: either a pirate base, or an underground base containing people who are seeking refuge from the pirates. Shepard can choose to visit whichever area Jack seems more likely to be.
If Jack was a paragon character in ME2 (didn't kill Aresh), then she will be found at the underground base, where she is protecting the colonists from the pirates. If Shepard visits the pirate base first, then he will be met with word from the base's captain that Jack has been disrupting his attempts to steal the colonists' resources – even though the captain bribed Jack to switch sides with information and resources. At that moment, pirates will attack the compound, and Jack and Shepard will have to fight them off. After the fight, Jack will agree to go with Shepard to the Shadow Base on the condition that they assault the pirates' base of operations, and that the colonist-refugees are taken to the Citadel. After helping take down the pirate base, Shepard finds a data pad in the pirate base which reveals the location of a Cerberus transmission relay outpost, which once decrypted can locate their Terminus bases.
If Jack was a renegade character in ME2, (killed Aresh/Shepard killed Aresh) she will be found at the pirate base, being about to perform a raid on the colonists in the underground base. The captain promised Jack information, a few men to help her, and the drug red sand (which Jack is now addicted to). Shepard has the option of convincing Jack to abandon the pirates and help protect the colonists, or helping the pirates raid the colonists.
If Jack and Shepard help raid the colonists, the pirate captain will give Shepard a data pad containing information on where to locate a Cerberus transmission outpost, which should lead to the location of other bases.
If Jack is convinced to abandon the pirates, they will attack Shepard, Jack, and company. After the raiders are defeated, Shepard can give stolen supplies back to the colonists, or send them to the Citadel, and discover a data pad leading to the location of a Cerberus transmission relay outpost which can reveal the locations of Cerberus bases in the Terminus.
If Shepard visits the colonists' base first, they will reveal that the pirates are preparing to raid them.
If Jack was not a loyal character in ME2, then she will automatically be renegade, and will stop at nothing to destroy Cerberus. She will have joined the pirate gang, and be preparing to raid the colonists – in exchange for information. If Shepard tries to convince her to turn on the pirates, she will disagree, claiming that Shepard wasn't willing to help her take down her base of Pragia on ME2. Shepard will have to kill all of the pirates and Jack (who has the abilities of an asari siren-husk). If Shepard agrees to join Jack in raiding the colonists, then afterwards, she will give him the information and join him on the Shadow Base.
If Shepard was aligned with Cerberus at the start of ME3, loyal Jack will need strong convincing to join him. Not-loyal Jack will be angry upon seeing Shepard-- and will attack him along with the other pirates.
Missions:
Once Jack is recruited, she is taken automatically to the Shadow Base, where she works on decrypting files and memorizing map layouts. From the base she will be available for interactions (romance and inquiries), and she is available for all of the “Three Heads of Cerberus”, “Shadow War”, and the side quest “Investigate ExoGeni”.
After Jack is convinced to join Shepard, she can be a temporary squad-mate for launching an attack on the Cerberus transmission outpost, located on a moon. During the mission, Shepard can ask what Jack has been up to since the SR2 defeated the Collectors, and Shepard was arrested by the Alliance. Jack (even love interest Jack) couldn't follow Shepard to Earth, because the Citadel Council has a bounty on her head – and the Alliance would've likely attempted to capture her.
[Note: I don't go into much detail about this Cerberus transmission outpost level because it is fairly straight forward. Jack and Shepard attack an outpost controlled by Cerberus to gain intel on their other bases of operations.]
Death:
Jack has the potential to die in a few other ways (not counting being killed by Shepard) in the story of Mass Effect 3.
At the end of the Heads of Cerberus campaign, Jack and Miranda will get into an argument over Cerberus's future – and their respective histories with the organization. Miranda will want to personally reform Cerberus, believing that they, like the Shadow Broker organization, can be turned into something better. Jack will want to destroy the terrorist cell, believing that its continual existence is an insult to all the people Cerberus has killed. This argument will escalate into Jack challenging Miranda's abilities as a protector of humanity when she can barely protect Oriana (paragon-Jack), or Jack making fun of Miranda's sterility (renegade-Jack). Miranda will punch Subject Zero in the face – which only delights Jack's longing's for a fight.
[Note: Yes, yes, I know. Harsh of Jack, right? Well, this is a very tension driven part of the game, and I wanted Jack and Miranda to finally have their foreshadowed fight. On top of that, I don't think it is above Jack to make fun of Miranda's near perfection in an attempt to really get to her. It is never revealed how Jack knows about Miranda's infertility issues]
While a paragon Shepard can choose to defuse the fight, renegade Shepard can allow the fight to occur – so long as it is off the Shadow Base. While some squad-mates will object, ultimately the decision lies with Shepard.
If Shepard condones the fight, Shepard, Jack and Miranda will take the kodiak to a [currently nameless] rocky desert world. Jack will jump out of the high flying shuttle (perhaps in an attempt to show her superior biotics), and land on the ground. When the shuttle lands, Miranda gets out and the two begin a furious fight.
If Jack is loyal, she can defeat a not-loyal Miranda, by summoning a small biotic tornado of rocks and dust, and hitting Miranda in the back with a large stone. Jack will then grab Miranda, and furiously use her biotic fists to punch Miranda's head in – covering herself in blood.
If Jack is not loyal but Miranda is, the exCerberus operative will kill Jack by breaking Jack's concentration to maintain a biotic tornado with a well placed biotic attack. Miranda will then shoot Jack out of the sky with an SMG, then Miranda will execute Jack by shooting her in the head – splattering herself with a small amount of blood.
If neither Jack nor Miranda is loyal, ME3 will randomly select one of them to die.
If both Jack and Miranda are loyal, then ME3 will randomly select on of them to die.
If either Jack or Miranda is a love interest to Shepard, the love interest will kill the other woman.
The other way for Jack to die in ME3 is for her to chase Kai Leng throughout Cronos Station during the fall of Cerberus.
If Jack is either not loyal, or Shepard allows her to chase after the assassin, Leng will mortally stab her when he lures her into TIM's office. She will die before the boss fight with Leng begins.
Jack can also be killed during the end game if she is a squad-mate, and if Shepard effective military strength is not strong enough.
Ironing out variables:
If Jack was not recruited in ME2 (via modding-- I guess?), she does not appear in ME3.
If Jack died in ME2, she does not appear in ME3. If She was romanced and died, Shepard can mourn her.
If Jack died in ME2, then finding out where the Cerberus transmission outpost can be handled by Miranda, Jacob, or (later in the game) Randall Ezno.
If Shepard had non-relationship sex with Jack in ME2, he can continue to have meaningless sex with her in ME3 – though he cannot romance her.