How do they compare in terms of prestige and job opportunities?
Depends on the field & where (geographically) you want to work.
But that would not be my first question when considering which university to choose between these three. Do you have offers from all 3?
Indeed. The actual uni experience and academic set up would be very different between the three. Especially between the two UK unis and an American college. Someone may do well or be happy in one and struggle or feel confined in another.