OK, start with the total of 64.
pick any three numbers as stated. I will to these: 20,20, 12
They add to 52, not enough, but wse need, lets see, 64-52 is 12 more. Dividing that equally, that means we add 4 to each of the numbers: 24,24,16 Which add to 64.
Again, another example: We pick three numbers, 30,30, 22 They add to 82, which is 18 too much. So divide 18 by three, reduce each of those numbers by six, which is 24,24, 16