All voltages in an electric circuit are potential differences between two points in the circuit. Therefore, if the voltage at the first red dot is denoted by Vr1 and the voltage at the blue dot is denoted by Vb, then when you measure the first red dot you are actually measuring (Vr1 - Vb) -- the difference in potential between these two points in the circuit.
Therefore, if you measure a first red dot and a second red dot and subtract the two measurements, what you are measuring is this:
(Vr1 - Vb) - (Vr2 - Vb) = Vr1 - Vb - Vr2 + Vb = Vr1 - Vr2
Therefore you are measuring the voltage between the two red dots. (These must be actual instantaneous voltages, not averages, when doing this calculation.)
Now of course, given the voltage between two red dots you can compute the voltage between a red dot and the green dot by means of phase angles and trigonometry, knowing that the phase angle is 120 degrees in a three phase system.