How do you make trigger box swap character to a new one randomly when touching trigger?

I am making a endless runner game and the character is either a cube, pyramid, or a sphere. The point of the game is to go through a wall that has shaped holes for each characters shape. When the shape goes through the right hole, it needs to turn into one of the other shapes randomly. How do I do that? (I am pretty new to UE4 btw)