The number is less than 20. it is an odd one-digit number. it is not the number of sides of a triangle. It can be divided by three. What is the number?

Its 9.

9 is one digit, less than twenty, divisible by three but not three itself, and it is odd. The only other possibility is 6 since it meets most of the criteria, except that its even not odd.

The answer is 9 because its an odd number, not the sides of a triangle, can be divided be 3, and is a single digit number.