We report the first measurement of the (e, e'p) three-body breakup reaction cross sections in helium-3 (He-3) and tritium (H-3) at large momentum transfer [< Q(2)> approximate to 1.9 (GeV/c)(2)] and x(B) > 1 kinematics, where the cross section should be sensitive to quasielastic (QE) scattering from single nucleons. The data cover missing momenta 40 <= p(miss) <= 500 MeV/c that, in the QE limit with no rescattering, equals the initial momentum of the probed nucleon. The measured cross sections are compared with state-of-the-art ab initio calculations. Overall good agreement, within +/- 20%, is observed between data and calculations for the full p(miss) range for H-3 and for 100 <= p(miss) <= 350 MeV/c for He-3. Including the effects of rescattering of the outgoing nucleon improves agreement with the data at p(miss) > 250 MeV/c and suggests contributions from charge-exchange (SCX) rescattering. The isoscalar sum of He-3 plus H-3, which is largely insensitive to SCX, is described by calculations to within the accuracy of the data over the entire p(miss) range. This validates current models of the ground state of the three-nucleon system up to very high initial nucleon momenta of 500 MeV/c.
