The behavior of the ABX3 hexagonal antiferromagnets in an external magnetic field is currently investigated both experimentally and theoretically. Usually the zero field in-plane configuration is the 120°-three sublattice configuration but elastic neutron scattering experiments on RbMnBr3 at low temperature revealed the existence of three phases: an incommensurate one for a magnetic fieldH<3T; two commensurate phases both characterized by a unit magnetic cell with 16 spins. The former occurs for 3T<H<4T, the latter forH>4T. Here we show that relevant features are entered by moderate competing in-plane interactions in the Heisenberg hexagonal model where antiferromagnetic chains are weakly coupled by in-plane interactions up to third neighbors. We recover some qualitative features observed in RbMnBr3 even if a quantitative and complete description of this compound is beyond the model we consider.