How do I calculate the point that is the least total distance from three other points?

Question

I'm trying to find a way to calculate the point at which the total distance from other points is at a minimum. In other words I'm trying to find a way to find the point at which the function $$d(x,y,z)=\sqrt{(x-x_1)^2+(y-y_1)^2}+\sqrt{(x-x_2)^2+(y-y_2)^2}+\sqrt{(x-x_3)^2+(y-y_3)^2}$$ is at an absolute minimum. Does such a solution exist?