I would answer 4, because I see that numbers are going up by 2 everytime they want and lower only by 1.

3,3,5,4,4,3,5,5,4,?,6

At the ? it could not then be 3 because next number is 6, next number can go up only by 2 get lower by 1 or doesn't change. So it must be 4 because 4+2 is 6.