Movie clip appears randomly and plays?

im trying to create a game like this

the only thing that i need to know is how to have a movieclip appear at the top of the screen, at a set y value and a random x value that is 0<x<550

it should be able to spawn 1 of 3 different movieclips and the rate at which they spawn should be tied into the level that you are at.

if you can even help me with 1 of these point then i would be grateful. thanks in advance