While I haven't used the Cityjet service, I have flown BA
and Flybe on the Avro and 146 respectively and they operated with a 3-3 configuration. It was tight on the window seats as there was limited head room. However if you seat yourself towards the back, not sure if this is the case on Cityjet but where the aircraft narrows there are sets of 2 seats. On the Flybe 142 it was 18AC DEF and 19AC.
Hope this helps