The magnetic phase transition in iron-nickel fcc alloys Fe₇₀Ni₃₀ doped by carbon (0.7% at.) is investigated using the three-dimensional analysis of the neutron depolarization (ND) and small-angle neutron scattering (SANS). The coexistence and the growth of two different scale magnetic correlations are observed in a paramagnetic phase as temperature closes to T_C. The usual critical fluctuations of a Lorentzian shape with size R_c<200 Å are found by the analysis of SANS intensity I(q). The ND analysis shows also presence of the large scale correlations (about 10³-10⁴ Å) with the "squared" Lorentzian shape. We attribute these large scale correlations to local variations of the Curie temperature T_C. The local T_C variations are described by the disorder parameters of the system: T_C, a spread of T_C variations ΔT_C, and characteristic size of the local areas R₀, where T_C variations occur. The ratio between the depth of the T_C variations (ΔT_C/T_C) and its characteristic correlation length R₀/a (a is a lattice constant) determines a scenario of the transition: "percolative" or "homogeneous" ones. At R₀/a≫ (ΔT_C/T_C)^-2/3 locally ordered ferromagnetic regions appear in paramagnetic phase and form a large percolative cluster. In this case the connectivity length of the ordered regions dominates at the correlation length of thermal fluctuations. Therefore this transition goes in accordance with the "percolative" scenario. All three parameters of the transition are obtained from the data of the three dimension analysis of the ND. From the temperature dependence of the magnetization, we found T_C=397±0.5 K and ΔT_C=4.55±0.05 K. From ND data we estimated the temperature independent characteristic size R₀ of the T_C variations. It is equal to 10⁴ Å.