There are many sets of three numbers that have products of 36.  The possible numbers are-1,2,3,4,9,12,36. However, only two sets have the same sum--2,2,9 and 1,6,6.--the number on the bus.  That's why John needed another clue after seeing the number (which we didn't have)  But, the statement itself is a clue for us.  If the bus number is not 13, John wouldn't have needed the third clue because another sum will provide a unique set of numbers with product of 36.

Thanks for playing.