3 images, with each their own mask.. So when 1 is rolled over, the mask increases in size, the others shrink accordingly. Each of the images/masks have to keep on checking each other's state, so an onEnterFrame event should be used. 
I tried doing this before, but the code sux. Hehe.
edit: or a simpler idea would be, for each mask/image movieclip:
onRollOver{
increase size;
decrease others size;
move others;
}
edit2: oh, and make sure you set the anchor point of the first on the left, the 2nd on the middle, and the third on the right. So when they scale, they will scale towards the center stage, like in the first link.