mosfet current mirror
Id is same in Q1, Q2 and Q3 (Current Mirror). Calculate it at Q1, using:
$$I_D=\frac{1}{2}\mu_nC_{ox}\frac{W}{L}(V_{GS}-V_{TH})^2$$ 
Apply the same equation for Id at Q3 and solve for its VGS.
Now, find Vs of Q3 using:
$$V_{GS} = V_G - V_S $$ where $$V_G = 0  $$
Now, find Vd of Q3 using: $$V_D = V_{DD} - I_DR_D $$
We have found Vd and Vs of Q3. Therefore:
$$V_{DS} = V_D- V_S$$