We study the Rabi model composed of three qubits coupled to a harmonicoscillator without involving the rotating-wave approximation. We show that theground state of the three-qubit Rabi model can be analytically treated by usingthe transformation method, and the transformed ground state agrees well withthe exactly numerical simulation under a wide range of qubit-oscillatorcoupling strengths for different detunings. We use the pairwise entanglement tocharacterize the ground-state entanglement between any two qubits and show thatit has an approximately quadratic dependence on the qubit-oscillator couplingstrength. Interestingly, we find that there is no qubit-qubit entanglement forthe ground state if the qubit-oscillator coupling strength is large enough.
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