Okay, I get
D_1=4+y^2, \; D_2=(\sqrt{2}-y)^2, \; D_3=4+y^2
Thus, the objective function, their sum, is D_t=(\sqrt{2}-y)^2+8+y^2
D_t'=6 y-2 \sqrt{2}
Which has a root at y=\frac{\sqrt{2}}{3}
Unfortunately, that is the reciprocal of the book's answer. Where did I mess up?