Well done on the offers. It's always going to be tough to choose between those three, as they really are very similar, especially when you add in the factor of graduate roles always being slightly random depending on the postings you get.
Salary/career progression is near-identical for equivalent grades of staff - there's a lot of turnover and interchange of staff between them so things stay fairly comparable and competitive. Likewise, training for all three is functional, internal stuff rather than developmental or qualification-based. IMHO, progression is too heavily based on time-served rather than meritocracy in these big IT firms, but you can always strike out on your own as a contractor and escape the hamster wheel.
There's not a lot to choose between on the info above (helpful, eh?). I'd pick the company which seems to have the most interesting contracts or seems to you to be winning the best work. If you've got the chance to pick a major contract or two to be based within then go for it as that'll be the biggest influence on the quality of the job.