At the time you leave with your UA flight, 2h should be sufficient, especially in biz. If everybody arrives 3h early, it will take at least 1h, but probably more to work down the queue and all those pax waiting for over an hour in line will make the flight. However check the min checkin time as some airports demand 2h or you may be denied joining the line.

For domestic, checkin lines can be long and in case of overbooking you are better off when you are early.