If you make the plan subscribable multiple times, but conditional to "no other active subscription of same plan" to avoid new subscriptions that would add the possibility to re-subscribe to another 90-days initial period.
However, if you want to have both conditions, as plans don't have separate conditions for initial purchase and for renewals:
a) if conditions for new 90 days purchase and for 30 days renewal are compatible (e.g. not reaching goal) you might be able to use the built-in active plans conditions
b) if all conditions for new 90 days purchase and for 30 days renewal are not compatible, you need to implement the condition using a CB FIeld that follows the plan's activation/deactivation with CBSubs CB fields integrations plugin and maybe auto-event CB plugin of incubator if needed, instead of the built-in plans conditions.