IIf() functions can be nested.
IIf(condition,TruePart,FalsePart)
You can replace TruePart and/or FalsePart with another IIf(). So -
IIf(condition1,IIf(Condition2,TruePart2,FalsePart2),FalsePart1)
You could then replace FalsePart2 (for example) with another IIf(). I warn you though, this structure can be very difficult to read if it becomes too complex. In that case you are better off creating a public function where you can use Case statements or nested If's which will be easier to read and maintain.