We　study　the　Rabi　model　composed　of　three　qubits　coupled　to　a　harmonic　oscillator　without　involving　the　rotating-wave　approximation.　We　show　that　the　ground　state　of　the　three-qubit　Rabi　model　can　be　analytically　treated　by　using　the　transformation　method,　and　the　transformed　ground　state　agrees　well　with　the　exactly　numerical　simulation　under　a　wide　range　of　qubit-oscillator　coupling　strengths　for　different　detunings.　We　use　the　pairwise　entanglement　to　characterize　the　ground-state　entanglement　between　any　two　qubits　and　show　that　it　has　an　approximately　quadratic　dependence　on　the　qubit-oscillator　coupling　strength.　Interestingly,　we　find　that　there　is　no　qubit-qubit　entanglement　for　the　ground　state　if　the　qubit-oscillator　coupling　strength　is　large　enough.　©　2013　American　Physical　Society.