It's impossible to say for sure, as it often depends on who you get at the end of the phone. But the way I'd play it is to use the fact you've just left 3 early to your advantage.
Do you have a specific phone in mind? If so, phone O2 and say you've just left 3 early and you're in the market for a new contract, what deal can they do? See what they offer and tell them you'll be off to compare that to Voda and will phone back.
Then phone Voda, tell them the same and say what O2 have offered.
Then phone TMobile, tell them the same and say what O2 and Voda have offered.
Do this for all networks (other than 3!!

) and see where you end up. Then you could go back to O2 and say the others have offered a better deal, is their offer the best they could possibly do?