12/3 Wiring

That's called a multi-wire branch circuit (MWBC). They're discouraged anymore because of complications with AFCI and GFCI. For instance you will not be able to fit AFCI on this circuit very easily.

First, refrigerators do not need GFCI protection unless they are in a garage or basement. (and even then, you should ask your AHJ for a waiver of that requirement). Do not put fridges on GFCI protection. Ground them properly. Fridges are simply not a use-case for GFCI (the hot components are highly inaccessible and you're not likely to drop your fridge in your sink), many refrigerators have trouble tripping GFCI, and GFCI "safety" trips actually *add* a serious food-safety risk. So it is not so simple as "OMG GFCI everything 4 the saeftyzzzz!!!!!111"

So your plan to put a GFCI recp *past* the refrigerator is perfectly fine.

Second, you can't use separate breakers *for so many reasons*. You are required to use a 2-pole breaker. There are other ways, but if you don't fully understand the importance of phasing MWBCs correctly, it's best just to play it safe and use a 2-pole breaker. Note that a 2-pole breaker has its two handles factory tied. if the handles move independently, you may have a duplex, and that creates a very dangerous situation.

The purpose is common maintenance disconnect (and side effect, to force you to phase it correctly). Common trip is not required. Some arrangements (e.g. handle ties or "non-common-trip" breakers) do not *guarantee* common trip, but it's still very likely to happen.