How few stats is too few?

I've been putting together yet another fantasy hack of Apocalypse World (mainly as a theoretical exercise for my own enjoyment, since I know there are more than a few out there already), and I started out with four stats roughly corresponding to Strength/Dexterity/Intelligence/Charisma, but then I got to thinking... the two physical stats seemed to overlap in uses enough that it'd be simpler to streamline the system down to three stats, basically Body/Mind/Soul, largely akin to Hard/Sharp/Hot from AW.

Has anyone ever tried or tested an AW variant with only three stats before? My main concern is that it's too straightforward, and players will have fewer options for customizing/distinguishing their characters.

Monsterhearts and Sagas of the Icelanders use four, and they work just fine.  I imagine three would work okay as well (though If you're using the highlighted stats mechanics, I'd only highlight one stat).  I'm not to keen on body/mind/soul, though--those words don't tell us about the character in the way that cool/hard/hot/sharp/weird, cold/hot/volatile/dark, or versed/young/gendered/wyrd do.

What are your moves?  How well your stats work is really a function of how they interact with the moves--in particular, the basic moves.

I was thinking of giving out XP as incentives on certain moves and/or on a 6 or lower, Dungeon World style, so no stat highlighting to worry about. Also not actually planning on calling them Body/Mind/Soul, just thought that would be a comparison that'd be easy to pick up on for this thread; my first, four-stat concept was Might/Finesse/Wits/Spirit, and it's Finesse I'm thinking of dropping and merging into the others.

Basic moves are fairly conventional for an AW variant: Might gets slightly modified versions of Seize by Force and the Protect move from Monster of the Week (exact names still to be finalized, of course; just working out the framework), Wits gets Read a Sitch, Read a Person, and possibly one more (considering two Read a Sitch variants, one more for investigation, and the other for tactical awareness and the like in combat), Spirit gets Manipulate and maybe a rally ability that gives another player a bonus to their next roll (take +Bond forward, maybe, depending on how I decide to handle Hx/Bonds; still up in the air, though that's not what I'm wondering about right here). Like Dungeon World, instead of an Act Under Fire with a specific stat, I was planning on using a Defy Danger-esque move, using whichever stat is most appropriate for the action the player takes.

I did also write an Evade basic move using Finesse (which, sure enough, is about evading attacks), but even at the time, I realized Defy Danger basically covered that type of action already, I just didn't want to leave Finesse out in the cold as the only stat without an exclusive basic move, which was the first thing that set off warning bells that the stat was looking superfluous in my model.

Anyway, while working out the playbooks and their moves, I've been trying to give each playbook a main stat that they'll tend to favor, but also sprinkling in options that are based on one or two of the other stats. This was when I started to notice that I really didn't have anything else for Finesse besides the two playbooks that were focused on it: a stealthy thief type (main inspiration being Garrett from the Thief series), and a flamboyant swashbuckler (any Errol Flynn character), and many of their moves were variations on things that could plausibly be covered by one of the other three stats.

So I'm feeling relatively confident dropping Finesse is the right thing to do, what I'm concerned about now is whether having only three stats will eliminate choices and make there only one or two "obvious" ways for players to build their characters.

I had an identical struggle while I was working on my Ars Magica hack. After a bit of playtesting I decided to add the Dexterity stat back in (called Quick in Wizard World). The primary reason was related to your observation about differentiation. Players felt pretty unsatisfied with a single stat that made their character tougher, faster, stronger and better with both melee and ranged attacks. Now, if you're going for a highly abstract style of combat where fights are intended to be resolved with a single roll, you're probably OK with the amalgamation. Otherwise, I think there is definite value in separating fast from strong & tough.