With 3 FA's the lead sits in front in order to be closer to the cockpit and therefore covers the 1L door. It then goes in a horseshoe pattern with #2 (FC) at 2L, #3 (YC) covering 2R and if there's a #4 (FC or YC as needed), back up at 1R. This way, the 2 rear doors are always covered since there are technically more PAX that would potentially exit using those two doors (being that the less dense FC is in front and the over-wing exits are more towards the front). With larger AC, as FA's are added, the same theory is generally applied, trying to cover the exits in their order of usefulness.

It's really not a factor that the FC attendant sits in back, it only takes a few seconds to walk to the front cabin. It's a matter of when the FA's get up to start service.