A thought I had regarding 3 pointers and the Celtics:
While the math works out to where 3 pointers are generally just better than midrange shots, a shot right at the rim or a dunk is still the best shot in the game when it's open. Teams that are able to leverage 3s really well and actually win a championship off them like the peak Warriors aren't just shooting 3s. They're using those 3s to open up a lot of space for people to get to the rim. You can't clog the paint and deny all sorts of drives and dunks if Steph really can just hit his shots from wherever on the court, you have to guard him once he crosses half court and thus he can take players out of the defensive action with his mere presence. Steph in particular exploits this by running around a lot because he's a very skilled off ball player and his cardio is great. Either he gets open and hits his shot, or his defender is getting tired and can't help onto other players driving for the rim.
Where the Celtics falter is that they're forgetting this second part. If you get a wide open 3, great, but spacing is still ultimately in service of opening up that space to make rim attempts easier. It's inherently more unstable to rely on 3 point shooting because of shooting variance that affects everyone. If it's an off shooting night but you can still get the defense to respect it enough to pull them out to the 3 point line, you need to be able to recognize this and leverage that created space to go for shots at the rim. You can't forget what you're trying to create space for in the first place.
Basically, the difference with the 3s and the midrange shots is as follows:
Midrange: A shot at the rim is ideal, the paint is clogged and I can't get to the rim, therefore I'll try to get to where I can to make my shot.
3 Point: A shot at the rim is ideal, the paint is clogged and I can't get to the rim, therefore I'll try and get open for a 3 and if they follow me out to the line, the paint is no longer clogged and my teammate can get a shot at the rim. If I'm not followed, open 3 pointer.