The binding energies of H3 and He3 are calculated assuming two-body forces of exponential type with hard cores, radii of which are taken equal for both the spin singlet and the triplet states. The outside potentials are supposed charge-independent, and are chosen so as to fit the nuclear two-body data in the low energy region. The following results have been obtained concerning the effects of hard core: 1) it diminishes the binding energies of H3 and He3 considerably, and 2) the wave function is pushed out so that the Coulomb energy of He3, which is too large without hard core, is reasonably reduced to the experimental value for the difference between the binding energies of H3 and He3.