What is the sin of 3 pi?

The sine of (3\pi) is 0. This is because (3\pi) corresponds to a point on the unit circle where the angle is a multiple of (2\pi) (specifically, (3\pi = 2\pi + \pi)), and the sine of any integer multiple of (2\pi) is always 0. Thus, (\sin(3\pi) = 0).