Hmm. I think I'd suggest a houserule for Rapid Fire with Throwing, here - say that if all the objects you're throwing are small enough to fit into your hand, and if all of them are being thrown at the same target, you can halve the normal Rapid Strike penalty. So throwing 3 grenades like that would be -9, and you can throw at a target area instead of a person for +4, and probably Aim for a couple seconds too. Then make three attack rolls at the final modifier, and apply scatter on a miss, and you'll probably have results that aren't too far off from reality.