Load Transparent movies

Ok, I know this is probally has a simple answer. but i cant figure it out. i have 3 buttons each controling a different movie clip. but the effect i want is when you click a button the assigned movie clip loads ontop of the other. basically this is because the movie clips are alpha tweens and i need that transition effect.

Im not sure where to put the mc’s either (could this be the problem??)

is there a more simple way to do this?
Thanks in advance for all your help.

i think what you have to do is keep all the mcs on the same layer and swap depth between each, or assign a certain depth so that one is ontop of the others…