sin^2x + sin^2(x+pi/3) + sin^2(x-pi/3) = 3/2
Refer the link for the answer similar to your query:
https://www.meritnation.com/ask-answer/question/prove-that-cos2x-cos2-x-pi-3-cos2-x-pi-3-3-2prove-t/trigonometric-functions/6791645
https://www.meritnation.com/ask-answer/question/prove-that-cos2x-cos2-x-pi-3-cos2-x-pi-3-3-2prove-t/trigonometric-functions/6791645