here's the story I tell everyone in embedded, who is new to the field.
you want 3 boot partitions. 2 for flash and 1 for ROM.
your update toggles between partA and partB. you cant write to partC, its minimal and in rom. its there in case both A and B are not bootable (watchdog based).
I worked at a company that made radios that were for ISP's to use, tower to tower. a tower climb can cost $10k. you do NOT want to have to hire someone just to reboot or fix bad firmware. we learned that the cost of 3 partitions was worth it.
I used to work at car companies and guess what: they only have 2 boot partitions (at most) and they never have that rom partition. I've told several of them and they dont seem to see the cost benefit. (shrug)
but just be aware, if you want the job done right, have 3 partitions and one should be read-only so that you can always always recover to something; then use that to reflash A and or B.