I'm not saying your answer is wrong, but you forgot about the sum for sin.
\(\displaystyle S_n \le \frac{3}{n}\sum_{k=1}^{n}(1) = 3\)
The fact that it converges is due to sin(1+3k/n) always being positive except for when 1+3k/n > pi, but regardless as n gets bigger, for the values in the first quadrant alone, you can see that the sum is increasing and bounded above by 3.