Hi masudyo,
The Excel Sin() function expects the input to be in Radians, but you appear to be using Degrees. You must therefore convert your Degrees to Radians by multiplying by Pi/180.
Assuming your ‘x’ value is in cell e2, using the formula:
=SIN(E2*PI()/180)+SIN((E2+120)*PI()/180)+SIN((E2+240)*PI()/180) works. It always evaluates to 0 no matter what value you put in cell e2.