Im learning how to use scripting in blender. I would like to know how could I make a certain object to appear only in a certain frame.

I have been searching this for a long time and I still can´t manage to do it...

Initially I have two primitive_cubes moving around. At the frame x, a third cube starts to move. The problem is, the third cube is all the time visible, and I want him to appear only in the frame he starts moving...