Multiples of a number are the numbers which are exactly divisible by the given number.
Multiple of 12 = 12, 24, 36, 48, 60, 72, 84, 96, 108, 120, ………
Multiple of 18 = 18, 36, 54, 72, 90, 108, 126 ………………………………
Thus, first three common multiples of 12 and 18 = 36, 72, 108