  A search for new physics using three-lepton (trilepton) data collected with
the CDF II detector and corresponding to an integrated luminosity of 976 pb-1
is presented. The standard model predicts a low rate of trilepton events, which
makes some supersymmetric processes, such as chargino-neutralino production,
measurable in this channel. The mu+mu+l signature is investigated, where l is
an electron or a muon, with the additional requirement of large missing
transverse energy. In this analysis, the lepton transverse momenta with respect
to the beam direction (pT) are as low as 5 GeV/c, a selection that improves the
sensitivity to particles which are light as well as to ones which result in
leptonically decaying tau leptons. At the same time, this low-p_T selection
presents additional challenges due to the non-negligible heavy-quark background
at low lepton momenta. This background is measured with an innovative technique
using experimental data. Several dimuon and trilepton control regions are
investigated, and good agreement between experimental results and
standard-model predictions is observed. In the signal region, we observe one
three-muon event and expect 0.4+/-0.1 mu+mu+l events
