Oh, well, the thing with Jin is that when that episode aired, a bunch of folks were floating theories that she might be a Dai Li agent (like all of those creepy brainwashed Joo Dees) checking "Li" and his uncle out -- which doesn't seem to have panned out in canon, but made for some interesting fics. So in some versions, her magically expanding cleavage was a very deliberate act of wardrobe adjustment on her part, meant to distract the boys wih her assets while she tried to get them to slip up and reveal their true identities. ;)

Suki, it's hard to tell since she almost always has armor on and you'd expect that would flatten things out a little bit -- but the one time we see her without it in Ba Sing Se she has a bit of a chest, but nothing near as curvy as Ty Lee or Jin -- not flat, but not in pain without a sports bra either (http://screenshots.avatarspiritmedia.net/212/105.jpg)...

And geeze, yeah, I sure didnt have boobs like Toph seems to have sprouted when I was twelve -- I *still* don't have them! *weeps, etc*