3 box lights is a bit much to light the screen. I would purchase a boom for 1 of the box lights to be above the subjects. This will help with shadowing around the subjects. Also need a couple or a few umbrella lights near the camera to light the subjects.

The box lights remove the shadows from the screen but are not brighter than the subject lighting. Subjects will stand or be at least 4 - 6 feet away from the screen.

Should do the trick. We use this setup all the time with our green screen photo shoots. We are now just getting into video green screen which should not be too much different.

Hope this helps