As you bump your way around the internet treehouse sites the big difference in building a treehouse as opposed to something on the ground is allowing the trees to move .
There are different ways to do this . What I did for the platform was to have Ruddick Welding make a bracket that one end of a beam could sit in and slide -attached the bracket to one tree and pinned the beam through at the other end
On the third tree I built a support Tee and attached that and then had the joists ride on top of all this creating the main framing for the Platform bits of the frame are hung off and corner braced back to the tree.
Below is a bunch of pictures that should explain what’s going on.