You can treat them as the sum of 2 phasors. Assuming they always have equal frequencies then it's a vector sum of the real and imaginary components.
Sum = Ax1*Ax2*exp(1j*(theta1+theta2-pi/2) +Ax1*Ax3*exp(1j*(theta3+theta2-pi/2)
Notice I shifted the waveforms to cosine to make the real and imaginary components work.
The magnitude and angle of the resulting wave are M=abs(sum) ThetaS = angle(sum) Y = M*cos(2*pi*50*t + ThetaS)
(Typing on mobile so please excuse if there's a typo or sign error)