  Gamma ray bursts (GRBs) have great advantages for their huge burst energies,
luminosities and high redshifts in probing the Universe. A few interesting
luminosity correlations of GRBs have been used to test cosmology models.
Especially, for a subsample of long GRBs with known redshifts and a plateau
phase in the afterglow, a correlation between the end time of the plateau phase
(in the GRB rest frame) and the corresponding X-ray luminosity has been found.
In this paper, we re-analyze the subsample and found that a significantly
tighter correlation exists when we add a third parameter, i.e. the isotropic
$\gamma$-ray energy release, into the consideration. Additionally, both long
and intermediate duration GRBs are consistent with the same three-parameter
correlation equation. It is argued that the new three-parameter correlation is
consistent with the hypothesis that the subsample of GRBs with a plateau phase
in the afterglow be associated with the birth of rapidly rotating magnetars,
and that the plateau be due to the continuous energy-injection from the
magnetar. It is suggested that the newly born millisecond magnetars associated
with GRBs might provide a good standard candle in the Universe.
