A search for the decay B^+ -> 3 mu nu is presented using LHCb data based on 4.7 fb^-1. The search is performed in a region where the minimum of
the two oppositely charged muon mass combinations is below 980 MeV/c. No signal is found and an preliminary upper limit of 1.4 x 10^-8 at 95% confidence level is set on the branching fraction.