Can u put a 3 mobile chip into any unblocked phone or must it be a 3 phone?

Answer
Unfortunately it must be a 3 phone, as i found out when my husband dropped his mobile into a muddy puddle. its a bit annoying, because all of the mobile phone shops ive been to, including the 3 shop itself, have stopped selling payg handsets. i am going to keep trying e-bay!