[en] A search for Supersymmetry in events with three leptons (electrons or muons) and missing transverse momentum is presented. The observation of a significant excess of events with three leptons in the final state with respect to the prediction of the Standard Model would be a hint of New Physics. A sample with an integrated luminosity of L=2.06 fb^-1 of proton-proton collision data at a centre-of-mass energy of √(s)=7 TeV delivered by the LHC and recorded by the ATLAS detector in 2011 is used. Special focus is placed on the composition of the Standard Model background and the measurement of misidentification rates of electrons and muons. The misidentification rates are determined from Monte Carlo simulated samples. The misidentification rates obtained for electrons yield values between ∝ 20% and ∝ 6%, depending on the transverse momentum and the pseudorapidity of the electrons, while for muons values between ∝ 42% and ∝ 10%, depending on the transverse momentum of the muons, are obtained. Two different selections are investigated, one selection vetoing events with the presence of Z bosons, the other selection requiring the presence of a Z boson. The observations of 32 events with an expectation of 26±5 events due to processes of the Standard Model and 95 events with an expectation of 72±15 events, respectively, are interpreted in a phenomenological Minimal Supersymmetric Standard Model and in simplified supersymmetric models. Limits are placed in the mass parameter space of these models.