How do you graph y= 1/2sin 3 x ?

A general sine graph of form $y = A \sin B x$ has amplitude A and period ${360}^{\circ} / B$.
So in this case, $y = \frac{1}{2} \sin 3 x$ has amplitude $\frac{1}{2}$ which is the maximum displacement from the x-axis, and period $T = {360}^{\circ} / 3 = {120}^{\circ} = \frac{2 \pi}{3} r a d$ and represents the distance on the x-axis for 1 complete cycle.