How modify this variable to 3 elements

i am using this variable to change the value in a field:

IF(IFnull(DATEDIFF(ADDDATE(IssuedDate, INTERVAL ExpirationTIme month),NOW()),datediff(ValidUntil,Now()))>0,'Active','Expired')

but my problem is that i need actually has 3 values, Active Expired and N/A,

N/A only will be show if ValidUntil is null

Maybe could be another way to do this..

any idea? thanksss in advance