We consider a model consisting of three two-level atoms in a heavily damped cavity. We show that the quantum-jump-based feedback can be used to generate a steady entangled state of three atoms against decoherence. When the feedback acts on just one of the atoms, it can protect a maximally entangled state of other two atoms. When the feedback acts on three atoms, by choosing appropriate parameters we can obtain a decoherence-free subspace spanned by two vectors, and by using quantum trajectory Monte Carlo wave function method we find that the maximally entangled state of three atoms in this decoherence-free subspace can be obtained for some specific initial conditions.