Pot's too small. Trees mostly too tall and not close enough together.
I stole this photo from an old post by Redwood Ryan (forgive me) --the trees are BC (needs a repot), Notice how close the trees are together. Those trees started out the same size as yours many years ago. The owner drastically pruned the root masses to fit together while the trees were young. That selective drastic root prune allowed them to get the trees placed more tightly together. BC can take that kind of thing, provided you keep the soil moist.