The statemment 3+3=6 serves as a/an to the conjecture that the sum of two odd numbers is an odd number.

The statement 3+3=6 serves as a disproof to the conjecture that the sum of two odd numbers is an odd number.
Odd numbers are numbers which can  NOT be divided evenly into groups of two.
6 can be divided by 2 (6:3=2), which means that the sum of two odd numbers is not an odd number. 
On the other hand, there is a statement that is always true (a theorem) about the sum of two odd numbers: The sum of two consecutive odd numbers is divisible by 4.