Solution
Consider adding three vectors of equal length to get zero resultant vector
choose ?1=0 , ?2 = 2?/3, ?3 = 4?/3
add any two => phase difference = 2?/3
amplitude = 2y1cos(??/2)=2y1cos(?/3)=y1
hence intensity is I1 => c)
y=y1sin(kx-?t+?1)+ y2sin(kx-?t+?2)+ y1sin(kx-?t+?3)