In three-dimensional O(N) models, we investigate the low-momentum be-havior of the two-point Green’s function G(x) in the critical region of thesymmetric phase. We consider physical systems whose criticality is charac-terized by a rotational-invariant fixed point. Several approaches are exploited,such as strong-coupling expansion of lattice non-linear O(N) σ models, 1/N -exp...