Without resorting to slang (ass, gut, box, nut, etc.), how many body parts can you name that have legitimate 3-letter names? I came up with 11.
arm ear eye gum hip jaw leg lid lip rib toe
What am I leaving out?
“Gut” isn’t slang, there’s a medical journal named Gut.
There’s “sac,” although unlike those above it requires an adjective (lacrimal, alveolar). I guess “bed” and “arc” would also follow this rule (nail bed, arc of Riolan). They don’t refer to a single body part category, like “ear” or “rib,” so I guess they don’t count.
There’s “fat,” whether that’s a body part or not is up to you.
Without going into acronyms (ACL), you probably have them all.
Thanks for the response. I seem to recall this issue coming up in a “serious” context some time ago. Not on The Dope, but on some other site or maybe even in a magazine. I do recall the “gut” issue being addressed before so your answer makes it a legitimate part of 12 such things. I agree with your assessment of “bed” and “arc” for the reasons you state.
As an aside/hijack, in my early days of writing haiku I tried to make them acrostic and the use of legitimate 3-letter words fit that need. It’s really amazing how many 3-letter words there are, both with and without slang and acronyms.
Speaking of which, my favorite acronym is TLA. 
pit
Pad
Web (between fingers/toes)
Orb (eye)?
zit!
And if that’s not enough: pus.
Bud (as in taste…)?
lbo?
nee?
fut?
hed?
bak?
noz?
han?
Does rod count?
(No, not THAT, ya pervert, as in rods and cones.)
It just might since it doesn’t need the additional term to indicate what type of rod you’re meaning (other than that one).
Sac, as in the lung’s air sacs.