In this case, you can use nested if  statement, present in excel 
here you need to put the second statement under another if statement and the third statement under else statement 
since it is put inside another if, we call it nested if statement 
example where you would want to add medium in your size listing .then you need to put this extra if in the statement.
=IF(column number="S","Small",IF(column number="M","Medium","Large"))