Normally 3 hours should be enough to make a connection though it can get busy.

I've flown UA from LHR and we landed at the farthest satellite terminal which has its own customs and immigration desks for connecting passengers, though I don't recall if its only for UA to UA connections. I've suggested to the mods that this thread be moved to the UA forums as they will have more experience at IAD for your situation. Especially for questions 2&3