IF A+B+C=Π prove that sin(3A)+sin(3B)+sin(3C)=-4cos(3A÷2)cos(3B÷2)cos(3C÷2)

Answer 1

You should start here, and it should be easy to adjust to using 3A, 3B, 3C.

https://socratic.org/questions/if-a-b-c-pi-then-prove-that-sina-sinb-sinc-4cos-a-2-cos-b-2-cos-c-2