How are your power ups organised ? Are they all the same object, with different animation/frame ? If so, you just have to spawn one power up, set its animation to "floor(random(3))+1" (witch will sent back 1, 2 or 3, so add a prefix before it) and you are done.
If it's different object, you will want to make a function "spawnPowerUp" that will start by doing a random, storing it in a local variable, and then will instance an object depending on the random. Example in this link.