  The effect of three-spin interaction k on thermal entanglement between
alternate qubits is studied using pairwise concurrence C and energy level
diagram. It is found that k breaks the symmetry about the effect of magnetic
field h on C. It shifts a dip structure and gradually effaces a boot structure
when |k|<|J| (J is spin exchange coupling). A peak with C maintains 1 appears
and expands, elbowing the dip backwards when |k|>|J|. A sudden change of the
concurrence occurs around |k|=|J|, h=-k. Similar conclusions about
nearest-neighbor qubits are directly given.
