  We investigate two types of genuine three-qubit entanglement, known as the
Greenberger-Horne-Zeilinger(GHZ) and W states, in a macroscopic quantum system.
Superconducting flux qubits are considered theoretically in order to generate
such states. A phase coupling is proposed to offer enough strength of
interactions between qubits. While an excited state can be the W state, the GHZ
state is formed at the ground state of the three flux qubits. The GHZ and W
states are shown to be robust against external flux fluctuations for feasible
experimental realizations.
