Drill holes at the top and bottom of each of the 4 corners, of each long piece. (obviously, you will have 12 holes 4/sheet).
Now use zip ties to fasten. Don't pull too hard or it will buckle once you join all the pieces together.
GOAL >> The object is to join all the pieces together.
As you work, I suggest checking the fit under the tree, each time you join an additional piece on.
Each top and bottom should match, and look uniform, as though they are ONE piece.
Continuity and flow of look matter here.
For the last piece (the third), makes sure it too is uniform and flush with the others.
You will have an overlap on the inside (no one will see it no worries).
The overlap is good as the plastic is a bit more flexible, but when reinforced by itself (the other pieces) it is stronger.
Trim the zip ties and I suggest adding a bit of glue to the areas between the zip ties.
Totally optional