The distance AB is twice the height of a triangle:
(Use Pythagorean theorem)
Sorry, maybe my question wasn't clear enough. I am looking for the distance from a [random] candidate (3), with known distance to (1) and (2), to the beacons (A) and (B).
[ I was still able to calculate the distance AB myself ;-) ]
By the way, the triangle is not equilateral (or only by coincidence). The distance between the beacons (1) and (2) can vary.
(I'll update the picture to give a clearer view on the situation).