subscription progression

How to set up subscriptions for this payment plan? $15/month (first 3 months mandatory , month to month at will after that) . Don't want them to have to pay a lump sum upfront for the 3 months but per month, then renew at less amount month to month.

I'm thinking single payments gateway is so it's up to them to renew whenever ?

Basically I want to charge a finite payment per month then progress to serial per month on the same subscription. Or is it possible to have only members of serial subscription A progress to serial subscription B bc it says it's starts over? Should I not put the same level twice on the same subscription progression ?