Theoretical studies of microdischarges are less advanced than their experimental counterparts. The knowledge of fundamental mechanisms that account for dc microplasma generation and that control the remarkable stability of the discharge at high gas pressure is still incomplete. Here we present results from modelling of 3-electrode microdischarge configuration consisting of a microhollow cathode discharge (MHCD) (used as plasma cathode) and a microcathode sustained discharge (MCSD) (maintained in the volume between the plasma cathode and a third positive electrode). It is shown that the MCSD is similar to a positive plasma column and that it can be modelled separately from the MHCD, thereby eliminating the complexity inherent in modelling the cathode regions. This approach allows us to concentrate more on the plasma chemistry and to use the model as an efficient diagnostic tool.