3 months membership payable at once

Hello Support.

We have 3 different payment Subscription levels, Platinum, Silver, Gold. Each of these levels can be paid yearly or monthly.

I have set them up (all six of them) as "Serial".

Question: On the three MONTHLY memberships, I want them to pay the first 3 months upfront while signing up. Then at the end of the third month, it will charge them monthly.

How do I set this up...??

Thanx in advance.