Abstract: We search for the decays of $B$ mesons to charmless final states involving a baryon and an anti-baryon using 9.2 fb -1 of e + e - annihilation data taken on the?(4s) resonance with the CLEO II and CLEO II. V detectors, and 6.2 fb -1 from CLEO III. We perform an extended maximum likelihood analysis of the data to determine signal yields. We report an observation of the decay mode B - -> p anti-p K - and measure its branching fraction to be (5.88 +2.86 -2.21 (stat.) +1.12 -0.19 (syst.)) x 10 -6 . We find no statistically significant signals for our other modes and establish 90% C.L. upper limits of BR(B -> p anti-p) 1.4 x 10 -6, BR(B - anti-p?) 1.5 x 10 -6, BR(B -? anti-?) 1.2 x 10 -6, and BR(B - anti-p?p +)

This thesis discusses different aspects of the analysis of charmless three-body B-meson decays. A study of a method of extracting the weak phase gamma using five decay modes under the assumption of flavour-SU(3) symmetry is performed. This study is carried out using BABAR-analysis results and consists in a complete proof of principle, including fully propagated uncertainties. Six possible solutions are obtained with an uncertainty below 11 degrees. It is also found that, when averaged over the entire Dalitz plane, the effect of SU(3) breaking on the analysis is only at the percent level. A second analysis presented in the manuscipt consists in the measurement of the branching fractions of the B0(s)->KShh' modes, where h(') represents a pion or a kaon. It uses data collected by the LHCb detector during run I and a part of run II, corresponding to an integrated luminosity of almost 5/fb. The B0s->KSKK decay mode is observed for the first time, with a significance of almost 7 standard deviations; the B0s->KSKK branching fraction with respect to B0->KSpipi is extracted. This analysis paves the road to the measurements of the branching fractions of the six B0(s)->KShh' decay modes with the whole run I and run II LHCb dataset.