In this work, a unique network core-shell structure based poly(Pyrrole-co-O-Toluidine) (PPCOT) - NiFe₂O₄ (NF) nanoparticles (NPs) and decorated with cross-linked single-walled carbon nanotubes (C-SWCNTs) has been synthesized as an electrochemical sensor for Fe³⁺ ions. The results revealed that the electrochemical properties and sensitivity of the ternary PPCOT/NF/C-SWCNT nanocomposites (NCs) were considerably better than the individual properties of copolymer PPCOT, binary PPCOT/NF NCs or NF NPs. The improved properties were attributed to the 3D structure of C-SWCNTs, which offer a large specific surface area that improves the electrical conductivity of the ternary PPCOT/NF/C-SWCNT NCs. DFT calculations for the heat of formation of monomers (PT, PP and TT) were also carried out. The heats of formation of the standard states of the monomers are exothermic and spontaneous at all temperatures. Here, the NCs of the synthesized 3D network core-shell were coated on a working electrode of an electrochemical sensor; the current collector is a glassy carbon electrode. The fabricated sensor is displayed good sensitivity and lower limit of detection of 11.0158 μA μM⁻¹ cm⁻² and 97.08 ± 4.85 pM, respectively. By the increasing of PPCOT, it is significantly increased the electrochemical interactions among the matrices of PPCOT, C-SWCNTs and NF. The Fe³⁺ sensor showed a preference towards Fe³⁺, which was linear for concentrations ranging from 0.1 nM to 0.01 mM. Moreover, the sensor’s analytical parameters, such as reproducibility and response time, were outstanding. Thus, the assembled sensor is highly effective and useful for Fe³⁺ ions detection in real environmental samples.