if your button exists in the 2nd and third frame, you can assign your movieclip's visible (or _visible) property to true.
if your button only exisits on the first frame, you could have your button assign a variable to true, if it's clicked. then use that variable when your movieclips exist (in the 2nd frame) to assign their visible property to be that variable's value.
How about showing a script on how to have the button assign a varible.
To have the variable approach work you'd need to have an actions layer that extends across all the frames that involve these various objects. In the first frame of that layer you would declare the variable. In AS2 and AS3, the code for the variable you'd use could be the same... let's say you name it "showMC"... so in frame 1 of that actions layer...
Then, in your button's event handler function you would set the value of showMC to be true...
showMC = true;
Hopefully you know how to code a button to work, but if not, indicate which version of actionscript you are using.
Then, in frame 2 where the mc is, if the instance name of the movie is mcName, then you would have...
//use one or the other below, but not both
mcName._visible = true; // for AS2
mcName.visible = true; // for AS3
I would normally have that frame 2 code on a different actions layer that I use for individual frame code, keeping the one with the showMC variable unbroken by other keyframes.