If you are building between 2-3 trees they will have to be big solid trees. I have built them with 2x12's with 4, 5inch lag screws into each tree and still had the trees move enough to break them apart before winters end.
My best results have come from building on 4 poles set in the ground 4 ft. or 2 poles with the platform leaning against the tree( like a ladder stand just bigger) and lagged to the tree. I would use a big tree too to lessen the movement from the wind. Then you can make it a 4x4 or 4x8 or whatever platform to build your walls on. Also an x braced house on poles with the poles leaning inward to create a larger base works to set on top of the ground. You can also build them on an old trailer and park them where you want.