Just put another IF in as follows:
=IF(B11="X",HLOOKUP(C14,List1,2,1),IF(B11="Y",HLOOKUP(C14,List2,2,1),HLOOKUP(C14,List3,2,1)))
This assumes that cell B11 can only have the values X,Y and Z. If you want to catch the error if B11 is equal to something else (eg A) then add another IF just as I've done above, and make the ELSE part of the IF statement display an error message.