  We present theoretical predictions for the hadroproduction of t tbar W+, t
tbar W- and t tbar Z at LHC as obtained by matching numerical computations at
NLO accuracy in QCD with Shower Monte Carlo programs. The calculation is
performed by PowHel, relying on the POWHEG-BOX framework, that allows for the
matching between the fixed order computation, with input of matrix elements
produced by the HELAC-NLO collection of event generators, and the Parton Shower
evolution, followed by hadronization and hadron decays as described by PYTHIA
and HERWIG. We focus on the dilepton and trilepton decay channels, studied
recently by the CMS Collaboration.
