What are the common mutiples of 3 and 12

The Least Common Multiple (LCM) of 3 and 12 is 12 so any number which is divisible by 12 is common multiple of them like:

12, 24, 36, 48, 60

Hope this helps!

3=3, 6, 9, 12, 15, 18, 21, 24, 27, 30, 33, 36, 39, 42, 45, 48, 51, 54, 57, 60, 63, 66, 69, 72, 75, 78, 81, 84, 87, 90, 93, 96, 99, 102, 105, 108, 111, 114, 117, 120, 123, 126, 129, 132, 135, 138, 141, 144, 147, 150, 153, 156, 159, 162, 165, 168, 171, 174, 177, 180, 183, 186, 189, 192, 195, 198, 201, 204, 207, 210, 213, 216, 
12=12, 24, 36, 48, 60, 72, 84, 96, 108, 120, 132, 144, 156, 168, 180, 192, 204, 216, 228, 240, 252, 264, 276, 288, 300, 312, 324, 336, 348, 360, 372, 384, 396, 408, 420, 432, 444, 456, 468, 480, 492, 504, 516, 528, 540, 552, 564, 576, 588, 600, 612, 624, 636, 648, 660, 672, 684, 696, 708, 720, 732, 744, 756, 768,

i am giving you an idea 
1. first add the multiples of 12 (24 ; 2+4=6,36;3+6=9)
2. if the summation of the nos becomes a multiplier of 3 it will be the common multiple of 12 and 3

hope you understand