In a kinematically complete experiment at the Mainz microtron MAMI, pion angular distributions of the 3 He(e,e'π + ) 3 H reaction have been measured in the excitation region of the Δ resonance to determine the longitudinal (L), transverse (T), and the LT interference part of the differential cross section. The data are described only after introducing self-energy modifications of the pion and Δ-isobar propagators. Using Chiral Perturbation Theory (ChPT) to extrapolate the pion self energy as inferred from the measurement on the mass shell, we deduce a reduction of the π + mass of Δm π + =-1.7 + 1 . 7 - 2 . 1 MeV/c 2 in the neutron-rich nuclear medium at a density of ρ=0.057 + 0 . 0 8 5 - 0 . 0 5 7 fm - 3 . Our data are consistent with the Δ self energy determined from measurements of π 0 photoproduction from 4 He and heavier nuclei.