name the imageMC’s: imageMC1, imageMC2, imageMC3 etc.
on the button place something like this:
on(release){
// if there are 3 imageMC's
myTarget = Math.round(Math.random(2)+1);
_root["imageMC"+myTarget].play();
}
Math.random(2) gives you a random value between 0 and 2 +1 makes this a number between 1 and 3. Math.round() rounds this value to the nearest integer, e.g. 2.132 will become 2
Note that in this case if myTarget would equal 2, then _root[“imageMC”+myTarget].play() would tell imageMC2 to start playing.