The sensitivity of light metal functionalized boron carbide (BC3) sheets towards selected carbonaceous gases like CO, CO2, and CH4 is investigated by using first principles density functional theory calculations. We find that functionalization with alkali (Li, Na, K) and alkaline earth metals (Be, Mg, Ca), is a useful strategy to improve the sensitivity of graphene-like BC3 towards the mentioned gases. A semiconductor-to-metal transformation of BC3 is observed upon the introduction of metal dopants. Gas molecules are adsorbed on the metallized BC3 through weak chemisorption, which is an ideal scenario for gas sensing under practical working conditions. We find that the adsorption energies (Eads) of CO molecule are found to be 1.71, 0.48, 0.34, 0.35, 0.96, and 0.84 eV on Be-, Li-, Na-, K-, Mg-, and Ca-doped BC3, respectively. Similarly, CO2 binds to Li-, Be-, Mg-, and Ca- doped BC3 with Eads of 0.54, 0.87, 0.61, and 0.43 eV, respectively. For CH4, an Eads value of 0.74 eV turns out to be the strongest in case of Be-BC3. Bader charge analysis divulges that the transfer of charges results in the adsorption mechanism of the gases to the metallized BC3. In addition to feasible Eads, change in the work function upon the adsorption of gas molecules further confirms good sensitivity of the metallized BC3 towards CO, CO2 and CH4. Based on our findings, we deduce that metal-doped BC3 is an excellent candidate for the efficient sensing of harmful pollutants.