Low-field (B=2-80G) DC magnetic susceptibility, χ, is investigated in ceramic La_1-xCa_xMn_1-yFe_yO₃ (LCMFO) samples with x=0.3. When y is increased from 0 to 0.1 the ferromagnetic (FM) Curie temperature, T_C, decreases from 259 to 107K, indicating growth of disorder by iron doping. A transformation of LCMFO from a weakly frustrated FM phase between y≈0-0.05 to a strongly frustrated mixed, FM and glassy, phase between y≈0.07-0.10 is revealed by measurements of magnetic irreversibility and long-time (up to t=10⁵s) relaxation of thermoremanent magnetization. Critical behavior of χ(T) obeying the scaling law χ^-1(T)-χ^-1(T_C)(T/T_C-1)^γ≡τ^γ is observed below τ_cr∼0.05-0.2 with γ=γ₁≈1.4 corresponding to a three-dimensional (3D) Heisenberg spin system. Above τ_cr, χ(T) can be fitted with the same scaling law as for τ<τ_cr, but with another value of γ=γ₂≈1.7, which characterizes a 3D percolation system. The change of γ upon lowering the temperature can be explained by the formation of percolation clusters from an assembly of hole-rich nanoscale particles of the second FM phase embedded into the host LCMFO matrix at T well above T_C. The magnetic moments, radii, and concentration of these particles are determined.