Here are ALL of the ways we came up with;
1) 6, 12, 18 - skip counting in 6s
2) 6 + 6 + 6 = 18 (repeated addition)
3) 3 + 3 + 3 + 3 + 3 + 3 = 18 (repeated addition)
4) 5 + 5 + 5 + 3 = 18
5) 3 + 3 3 + 3 3 + 3
6 + 6 + 6 = 18
6) 6 x 3 = 18
(6 groups of 3)
7) 3 x 3 = 9 and 3 x 3 = 9 so 2 x 9 = 18 (or 9 + 9 = 18)