Customer would like to have a Subscription that bills every 3 months, then switches to monthly after the first three months. This is not possible.
But you can set up a 3 month Trial, charge a Sign-Up Fee equal to three months, and then after that the monthly billing will start. This is essentially a 3 month Subscription that converts to a monthly Subscription after 3 months.
Charge $15 per month
3 Months == $45
Sign-up fee is $45
Due today is $45 and then $15/month in 3 months.