Currently, when an RM Series mixer is set to LCR Mode, the Center channel is not recorded in Capture. Capture has traditionally had the ability to record each individual mono input channel and then a single additional stereo track, normally used to record the Main L/R mix on the first pair of Auxiliary Firewire Inputs in the Auxiliary Inputs Router.
With the RM series mixers, when in LCR mode, Capture currently does not have the ability to add an additional track to record the Mono bus as your Center channel. While it is possible to record the Mono bus using an additional Auxiliary Firewire Input, it is not possible to do so in Capture specifically. This feature should be added in a future Capture update.
If you need to record the Mono bus, you should use Studio One (or your preferred DAW software) so that you have full access to record any and all Auxiliary Firewire Inputs. You can assign a pair of Auxiliary Firewire Inputs to record the Mono bus source in the Aux Routing tab of the Settings view in UC Surface.
An additional workaround for creating a mixdown when using LCR Mode and recording with Capture is to open your Capture project in Studio One, so that your Capture tracks are imported into a Studio One project, allowing you to mix them in Studio One in the traditional fashion.