Triple Quad Formula

Here is the Triple Quad Formula: $(Q_1+Q_2+Q_3)^2=2(Q_1^2+Q_2^2+Q_3^2)$ where $Q_n$ is a quadrance .

You have three points with three associated quadrances (i.e. square of the distance). Two of the quadrances are 1. The points are distinct.

What is the third quadrance?