Nickelate superconductors, R1−xAxNiO2(where R is a rare earth metal and A = Sr, Ca), experimentally discovered in 2019, exhibit many unexplained mysteries, such as the existence of a superconducting state withTc(up to 18 K) in thin films and yet absent in bulk materials. Another unexplained mystery of nickelates is their temperature-dependent upper critical field,Bc2(T), which can be nicely fitted to two-dimensional (2D) models; however, the deduced film thickness,dsc,GL, exceeds the physical film thickness,dsc, by a manifold. To address the latter, it should be noted that 2D models assume thatdscis less than the in-plane and out-of-plane ground-state coherence lengths,dsc<ξab(0)anddsc<ξc(0), respectively, and, in addition, that the inequalityξc(0)<ξab(0)satisfies. Analysis of the reported experimentalBc2(T)data showed that at least one of these conditions does not satisfy for R1-xAxNiO2films. This implies that nickelate films are not 2D superconductors, despite the superconducting state being observed only in thin films. Based on this, here we propose an analytical three-dimensional (3D) model for a global data fit of in-plane and out-of-planeBc2(T)in nickelates. The model is based on a heuristic expression for temperature-dependent coherence length anisotropy:γξ(T)=γξ(0)1−1a×TTc, wherea>1is a unitless free-fitting parameter. The proposed expression forγξ(T), perhaps, has a much broader application because it has been successfully applied to bulk pnictide and chalcogenide superconductors.