can i make a mask layer ?

Hi,

I'm trying to create a cut out area on one piece of footage so that i can see through to the video layer underneath. i have a scene with a television set in and i want to change the image on the televison screen.

I'm not sure if this is the most logical way, but so far this is what i have done, I have three layers

Video 1 - is footage of a scene in a lounge with a televison set in
video 2 - is a B/W bitmap exported of video 1 and edited in photoshop. i filled the tevelsion screen area in white and the background black (i could make this any color if it would be better) and then i resaved it
video 3 - is my new footage that i want to see on the television screen

so to recap.... on Video 2 - the bitmap.... the black area would be video 1, and the white area (the tv screen cut out) would be video 3.

Can anybody tell me how i would do this ?

Best, Shaun