If you have a node at the corner, you can connect three beams to that one node.

Tents with tubes I have seen have a diecast metal corner with three fingers to slip the three tubes on. In that way, the tubes become "welded" to the corner so they can carry bending moments.

If you want a tube to be connected in a way that is like a spherical joint, then you can do an End Release on that beam so that it doesn't pass any moments through that end.