  Traditionally gamma-ray bursts (GRBs) are classified in the $T_{90}$-hardness
ratio two-dimensional plane into long/soft and short/hard GRBs. In this paper,
we suggest to add the "amplitude" of GRB prompt emission as the third dimension
as a complementary criterion to classify GRBs, especially those of short
durations. We define three new parameters $f$, $f_{\rm eff}$ and $f_{\rm
eff,z}$ as ratios between the measured/simulated peak flux of a GRB/pseudo-GRB
and the flux background, and discuss the applications of these parameters to
GRB classification. We systematically derive these parameters to find that most
short GRBs are likely not "tip-of-iceberg" of long GRBs. However, one needs to
be cautious if a short GRB has a relatively small $f$ (e.g. $f<1.5$), since the
chance for an intrinsically long GRB to appear as a "disguised" short GRB is
higher. Based on avaialble data, we quantify the probability of a disguised
short GRB below a certain $f$ value is as $P (<f)\sim 0.78^{+0.71}_{-0.4}
f^{-4.33\pm 1.84}$. By progressively "moving" a long GRB to higher redshifts
through simulations, we also find that most long GRBs would show up as
rest-frame short GRBs above a certain redshift.
