Since I rewatched UNBREAKABLE and SPLIT the night before seeing GLASS, it's hard for me to council someone who hasn't seen those movies or hasn't seen them recently. It might be hard to track what's going on in GLASS, but then there is lots of exposition to help you through it. But what I liked was seeing moments, camera movements from the previous movies repeated in this one.
Also, UNBREAKABLE is really slow and hardly anything happens. Which isn't a bad thing to me--because it's a meditation--you sit with David Dunn's characters for a very long time. SPLIT is a bit faster pace, but it doesn't really go anywhere--it's another character study. So if critics saw both those movies, I don't understand why they would expect GLASS to be substantially different. It moves slowly, the characters don't go anywhere--it's like the other two movies.
I think it's significant that the cloverleaf has three leaves (not four, which would make it lucky), so if there's a name for the society then I think it would be the Black Three Leaf Clover. Although now that I think about it "I'm Looking Over a Four Leaf Clover" has lyrics that apply to the three movies.
I'm looking over a four leaf clover
That I overlooked before
First is the sunshine, the second is rain
Third is the roses that bloom in the lane
There's no need explaining
The one remaining is somebody I adore
I'm looking over a four leaf clover
That I overlooked before.
The Black Three Leaf Clover want us to overlook the fourth leaf.