An analysis of the decays of B--/+ -> DK -/+ and B--/+ -> D pi(-/+) is presented in which the D meson is reconstructed in the three-body final states K--/+pi(+/-)pi(0), pi(+)pi(-)pi(0) and K+K-pi(0). Using data from LHCb corresponding to an integrated luminosity of 3.0 fb(-1) of pp collisions, measurements of several CP observables are performed. First observations are obtained of the suppressed Atwood-Dunietz-Soni decay B--/+ -> pi K-+(+/-)pi(0)pi(-/+) and the quasi-Gronau-London-Wyler decay B--/+ -> K+K-pi(0)pi(-/+). The results are interpreted in the context of the unitarity triangle angle gamma and related parameters.