"How many months does john have before his drivers license expires?"
months_until_exp (an unstored calculation field, with result type 'Number') = Round ( ( Licence_exp_date - Get ( CurrentDate ) ) / 30.5 ; 0 )
"Also i would like to see if i can a notification when - if license is 90 days from expiry date"
For the other two cases, try an unstroed calculation set to type text:
Licence_exp_date - Get ( CurrentDate ) < 90 ; "Less than 3 months to go!" ;
Licence_exp_date - Get ( CurrentDate ) < 0 ; "Licence expired!" ;