You may be able to say
Class End Date AFTER {{system.date}} where the system date is Today.
Class End Date AFTER (the 90+ days from today), which you would have to enter each time.
But I suspect this isn't ideal.
So if you can set up a Formula Field in SFDC that says TRUE if Class End Date is 90 days or more from Today.
I do not believe you can do this in marketo.