3/12 = divide 3 to get the lowest number possible (In this case 1, so 3 divided by 3) and then divide the bigger number by same amount you've divided the smaller number (12 divided by 3 is 4, because 4 x 3 = 12). So our answer is 1/4. I'm too lazy to go in depth, but basically, 3 (4 times) is 12, so 3 takes up 1/4th of 12.
