How To Put Phone On 3 Way Can I Buy A 3 PAYG Phone And Put My 3 Contract Sim In It?

Can I buy a 3 PAYG phone and put my 3 contract sim in it? - how to put phone on 3 way

I have a 3 phone on a contract for 18 months, but I want a new phone, I can take a 3 Pay As You Go. Or do I find unlock phone and placed my SIM card.