In the present work, the magnetic properties of high-T_cBi_1.7Pb_0.3Sr₂Ca_2-xSm_xCu₃O₁₂ system (x = 0.03) have been investigated, for different temperatures below T_c. Substitution of Sm³⁺ for Ca²⁺ is found to change the superconducting properties of the system, despite the weak paramagnetic nature of the Sm³⁺ ion. The dc magnetization measurements performed on the compound reveal that the superconducting regions decrease with increasing temperature and magnetic field. The results are explained on the basis of a possible reduction of hole concentration with trivalent rare-earth ion substitution, and also on the basis of the magnetic nature of the substituted Sm³⁺ ions.