We　propose　an　efficient　scheme　for　generation　of　three-atom　singlet　state　in　a　bimodal　cavity　based　on　quantum　Zeno　dynamics　and　the　large　detuning　condition.　The　influence　of　decoherence　induced　by　cavity　decay　and　atomic　spontaneous　emission　is　also　discussed　by　numerical　calculation.　The　advantages　of　the　scheme　are　that　the　initial　input　states　of　atoms　are　not　entangled　and　the　fidelity　is　insensitive　to　cavity　decay　and　atomic　spontaneous　emission　due　to　no　exciting　the　cavity　fields　during　the　whole　evolution　and　the　large　detuning　condition.