1. Choose 7 people to go into the first group: C(21,7) ways
2. Choose 7 people to go into the second group from the remaining 14: C(14,7) ways
3. The last 7 go into the last group: 1 way = C(7,7)
4. Divide by 3! as the groups are not distinct/unique
So yes, you need to divide by 3!