I can't tell you why Remington decided to go with 3 rounds vs 4, but everyone else that does that does it for reasons #3, and #4 on your list.
All of the clays games such as skeet, trap, and sporting clays allow a max of 2 rounds in the mag and chamber, so there is no reason for the mag to hold more than 1 round for those games. There have been guns designed with a 1 round magazine to use for the clays games. The longer 4+ mag tubes negatively effect balance and are handicap for those games.
Having 3+1 capability is a compromise for hunting, and many, maybe most states limit you to 2+1 anyway. That is a requirement for waterfowl hunting and many states simplify the regs by requiring 2+1 for everything. Others let you load more for other game.
The combat shotgun games are much different. Even most of the guys who are using guns that came with 4+1 mags use extended tubes in order to load 7+ shells. While I don't know specifically about this gun, you can probably buy extended tubes if you want. They are available for most common shotguns.
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"If you're still doing things the same way you were doing them 10 years ago, you're doing it wrong"
Winston Churchill
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