This functionality isn't built in to eCart. You would have to write some custom code to get this working. One way I can think of getting something like this working is to first create a cart using eCart 5 with no payment gateway. Have everyone enter the site and checkout using this cart the first time they go to the site. Then create another checkout that's connected to a payment gateway. Then check each time they log in to see if the 3 months is over and if it is forward them to the second cart to charge them.