Party A turns five times and Party B turns seven times, so the number of teeth of Party A: the number of teeth of Party B = 7: 5 = 14: 10.

Similarly, the number of teeth of gear A can be obtained: the number of teeth of gear C = 2: 5 = 14: 35,

Therefore, the transmission ratio of a, b and c wheels is 14: 10: 35.

Therefore, the number of teeth of the three gears should be at least 14, 10 and 35 respectively.