Nonlinear ac magnetic susceptibility of three polycrystalline $NdBa_2Cu_3O_7, YBa_2Cu_3O_7$, $Bi_2Sr_2Ca_2Cu_3O$ superconductors has been studied to examine the intergrain contribution to magnetic properties at low field. Nonlinear ac susceptibility was measured as a function of temperature, ac magnetic field and dc magnetic field. The complicated dependence of the magnetic susceptibility on the applied field below 50 Oe is attributed to the critical state of trapped magnetic flux in the intergrains. Numerical results of calculations using a modified critical state model are in good agreement with the measured ac susceptibility data. We find the relation $J_c\sim H^{-n}$ for the dependence of the intergrain critical current density $J_c$ on the applied magnetic field H. The parameters obtained are n=2.5, 2.2 and 1.0 for $NdBa_2Cu_3O_7$, $YBa_2Cu_3O_7$ and $Bi_2Sr_2Ca_2Cu_3O$ respectively in conformity with the previous results of $YBa_2Cu_3O_7(n=1.8\sim2.2)$ and $Bi_2Sr_2Ca_2Cu_3O(n=0.7/sim1.3)$. The penetration fields of intergrain measured at 77K are 14.5 Oe, 2.45 Oe and 0.75 Oe for $YBa_2Cu_3O_7$,$NdBa_2Cu_3O_7$ and $Bi_2Sr_2Ca_2Cu_3O$ respectively. This means that the pinning force and critical current density of intergrain for $YBa_2Cu_3O_7$ are much larger than those of $NdBa_2Cu_3O_7$ and $Bi_2Sr_2Ca_2Cu_3O$.
