If sina+sinb+sinc=3 then find the value of cosa+cosb+cosc

Hii

the maximum value for sin is 1 and since you have three sin's adding up to 3, each of the sin's must be 1!
So A = B = C = 90 degrees or any angle coterminal with 90 degrees.

And the cos of 90 degrees is zero. So cos(A)+cos(B)+cos(C) = 0+0+0 = 0.