Maybe 3 letters in 6 guesses is too hard, due to the shortness of the word, the number of 3-letter words with common letters, and the overall number of combinations?
5 letters AND being forced to use real words to guess is important, I think, to being able to get to the word in 6 guesses. I admit I got a Wordle on the 2nd guess the other day, but that was remarkably lucky, I usually get there between 3 and 5 guesses.
Using openers like STARE, CLOUD and MINTY burn up 14 unique letters - all 6 vowels and most popular consonants in 3 guesses - I feel like, by that point, there is no way you cannot get it by 6 guesses. Logic plays a huge part in it after that.
Using 3 letter words doesn’t get enough letters out there in the first few guesses to hope to arrive, logically, at the word by the end of 6 guesses, as mentioned by
@SMcNeill, especially if the word chosen has a ton of first-letters that go with the last two - ?at, ?ar, ?en…there are loads of those. It becomes a real roll of the dice.
Maybe 5 letters is the sweet spot between “relying on luck” and “reasonable number of guesses required”?