In the figure, pulleys are smooth and strings are massless, m1 = 1 kg and  m2=13 kg To keep m3 at rest, mass m3 should be

m3 is at rest. Therefore,2T = m3 g                                                                                               …(i)Further, if m3 is at rest, then pulley P is also at rest.Writing equations of motion, m1g - T = m1a                                        …(ii)                                         T - m2g = m2a                                            …(iii)Solving Eqs. (i), (ii) and (iii), we get                                                  m3 = 1 kg