(a) 6 and 8

Multiples of 6 = 6, 12, 18, 24, 30, 36, 42, 48, 54, 60, 66, 72, 78

Multiples of 8 = 8, 16, 24, 32, 40, 48, 56, 64, 72, 80

So,

First three common multiples are = 24, 48 and 72

(b) 12 and 18

Multiples of 12 = 12, 24, 36, 48, 60, 72, 84, 96, 108, 120

Multiples of 18 = 18, 36, 54, 72, 90, 108, 126

First three common multiples = 36, 72 and 108