Suppressed production of high transverse momentum charmed mesons has been observed in heavy ion experiments at RHIC. Initial studies based on charm quark radiative energy loss in the produced quark-gluon plasma failed to explain the observed large suppression. More recent studies suggested
that charm quark energy loss due to elastic scattering was also important. We have reexamined these processes and further considered the effect due to charm quark scattering by two gluons via cgg -> cgg or two quarks via cqq -> cqq. For charm quark radiative energy loss, we evaluate all
lowest order diagrams without the usual assumption that scattered gluons or quarks are fixed in space. Because of the large number of diagrams involved in charm quark scattering by two gluons, an approximate calculation is carried out based on a modified Parke formula. For charm quark scattering by two quarks, its contribution can be evaluated exactly. It is found that contributions from these three-body processes to charm quark energy loss are sensitive to both the gluon screening mass and the thermal masses of gluons and quarks. With masses from thermal
QCD calculations, the charm quark energy loss due to three-body processes is small compared to that due to charm quark elastic and radiative scattering but not negligible. Including charm quark energy loss from all three processes in an expanding quark-gluon plasma, we have studied charm
suppression in heavy ion collisions at RHIC and compared the results with available experimental data.