OK Terry... that outta help.
You got every answer from 10 ft to 40 ft.
If you're talking about on "average" what a "beginner" should get out of a 7'6" 3wt... I would say perhaps 25-30 ft and still maintain a proper presentation with decent accuracy. With this type of outfit... timing is everything. Don't try to force the cast. Once you start trying to muscle out some distance, your cast will fall apart. Once you master this, a 40+ ft cast should be easy to do. Wind is another issue. Obviously if you tried to cast in East Texas this past Saturday... you got nothing. 3wts need a calm day for decent distance.
OK... here's the thing though. When I'm fishing a 3wt... I'm never in a situation where I need to make a 40 ft cast.