You are correct for the questions you answered.
For the last question, you do have to take into account keys being played in different octaves. You have to break up the cases this way:
1. All three of A, B, C are played and two others
2. Two of A, B, C, are played and three others
3. One of A, B, C are played and four others
For 1): cases
For 2): cases
For 3): cases
The answer is the sum of these three.