This investigation explores the permeation characteristics of hydrogen (H2), carbon dioxide (CO2), and oxygen (O2) using metal oxide@g-C3N4/polycarbonate (M@CN/PC) membranes. This study involves the testing of various metals, namely lithium (Li), magnesium (Mg), and silicon (Si), with different filler percentages of M@g-C3N4 (3% and 10%). The permeability assessments were carried out at room temperature under a 40 psi pressure. The characterization of fillers and membranes was conducted using Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), and differential scanning calorimetry (DSC). Furthermore, we have also measured the permeability of H2, CO2, and O2, and evaluated the selectivity ratios of H2/CO2, H2/O2, and CO2/O2. Both permeability and selectivity were found to increase with M@CN content.