Circle A contains all positive odd numbers; circle B contains all factors of 33, and circle C contains all prime numbers. How many numbers belong to all three circles?

3 and 11 are the only two because 33 only has four factors: 1,3,11,33.

1 - in some cases people say this is prime, but it is now widely considered to not be a prime number

3 - prime

11 - prime

33 - not prime, divisible by 3 and 11