the greatest multiple would be infinity
If you mean the greatest common factor:
45 = 3 * 3 * 5
63 = 3 * 3 * 7
81 = 3 * 3 * 3 * 3
they all have 3 * 3 in common
the greatest common factor is 3 * 3 = 9
if you mean the least common multiple:
45 = 3 * 3 * 5
63 = 3 * 3 * 7
81 = 3 * 3 * 3 * 3
the least common multiple is 3 * 3 * 3 * 3 * 5 * 7 because that has all the parts needed to make each of the three numbers
3 * 3 * 3 * 3 * 5 * 7 = 2835