AlphaFold 3's pair representation stores one distance estimate for every pair of tokens: the model's internal notion of a distance map (structural biology) built from pairwise attention (machine learning). Fix how far two residues each sit from a shared reference point, and the distance between those two residues is no longer free: geometry boxes it in. That constraint is exactly what AF3's triangle updates exploit.
Two distances don't fix the third, they only bound it, geometry does the rest.