  We further develop the constrained mass variable techniques to determine the
mass scale of invisible particles pair-produced at hadron colliders. We
introduce the constrained mass variable M_3C which provides an event-by-event
lower bound and upper bound to the mass scale given the two mass differences
between the lightest three new particle states. This variable is most
appropriate for short symmetric cascade decays involving two-body decays and
on-shell intermediate states which end in standard-model particles and two
dark-matter particles. An important feature of the constrained mass variables
is that they do not rely simply on the position of the end point but use the
additional information contained in events which lie far from the end point. To
demonstrate our method we study the supersymmetric model SPS 1a. We select cuts
to study events with two Neutralino_2 each of which decays to Neutralino_1, and
two opposite-sign same-flavor (OSSF) charged leptons through an intermediate
on-shell slepton. We find that with 300 fb^-1 of integrated luminosity the
invisible-particle mass can be measured to M=96.4 +/- 2.4 GeV. Combining fits
to the shape of the M_3C constrained mass variable distribution with the max
m_ll edge fixes the mass differences to +/- 0.2 GeV.
