  We propose a new entanglement measure to quantify three qubits entanglement
in terms of negativity. A monogamy inequality analogous to
Coffman-Kundu-Wootters (CKW) inequality is established. This consequently leads
to a definition of residual entanglement, which is referred to as three-$\pi$
in order to distinguish from three-tangle. The three-$\pi$ is proved to be a
natural entanglement measure. By contrast to the three-tangle, it is shown that
the three-$\pi$ always gives greater than zero values for $W$ and GHZ classes,
implying there always exists three-way entanglement for them, and three-tangle
generally underestimates three-way entanglement of a given system. This
investigation will offer an alternative tool to understand genuine multipartite
entanglement.
