GeV γ-rays detected with the large area telescope on board the Fermi Gamma-ray space telescope in the direction of HB21,MSH 17-39 and G337.0-0.1 have been recently reported.The three supeova remnants (SNRs)show interactions with molecular clouds,and they are effective gamma-ray emitters as the relativistic protons accelerated by the SNR shocks inelastically colliding with the dense gas in the clouds.The origin of the observed γ-rays for the three remnants is investigated in the scenario of the diffusive shock acceleration.In the model,a part of the SNR shock transmits into the nearby molecular clouds,and the shock velocity is greatly reduced.As a result,a shock with a relatively low Alfvén Mach number is generated,and the spectra of the accelerated protons and theγ-ray photons produced via proton-proton interaction can be obtained.The results show that the observed γ-ray spectra for the three SNRs interacting with the molecular clouds can be reproduced.It can be concluded that the hadronic origin of the γ-rays for the three SNRs is approved,and the ability of SNR shocks to accelerate protons is also supported.