Forget the Canon Strobes, they're not going to give you anywhere near enough power to light up 170 people at night.
Take the three 320 watt strobes, and get some 60 inch silvered umbrellas. Put one right behind the camera, put the others to the left and right of camera, at least 10 to 15 feet away from the camera location. I'd turn them all up as far as they'll go. With that theatre, you're going to have to be careful that you don't shadow the group by putting them too far under the Marquis.
If you have the lights high enough up, (like 10 to 12 feet), evenly spaced apart, probably about 40 - 50 feet back of the group, and with the same amount of power, you should be able to spread the light around to the point where there shouldn't be any shadows.
David Buzzard's Technical Blog