So you just want a SIM to port out to another network, and then port back in to Three? To move numbers from one contract to another?
I know for a fact that you can use Tesco Mobile SIMs to do this. They don't require top-up for port in + port out. I know because I've done it several times for customers.
As a reminder, porting out and back in is only a workaround for networks that don't allow PAYM->PAYM number transfers (also called migrations). I'd recommend finding out first if your network can transfer numbers internally before you use this approach.
Ideally you could just take the new contract and ask Three to move your number across.
Hope this helps. And hope you're not talking about something else!