Bristol is more lively and I trust them more as an engineering faculty, Southampton is quite well known for overpromising and upselling what they have.
Out of the three I would probably say Bristol/Sheffield>Southampton, with the choice between Bristol and Sheffield being based on your preference of course structure. Bristol's course is closer to the standard format whereas Sheffield's goes off the rails quite a bit with its specialisms which can be great for some but awful for others.
Probably also worth adding that travel links could be an important consideration, Sheffield has better access to a major international airport than Southampton which has slightly better access than Bristol.