Nope, it's part of the initial AH3 game installation and is on your harddrive when you install the game.
There's only one problem. Patch 15 broke the TE so we all need to wait until the next patch fix it.
Advice:
Start with a small practice terrain before you try that monster creation you expect everyone to love,
and,
never upload a terrain to the server until you are sure it is one you'll finish for an arena,
and,
the name you use should be the final name because even Skuzzy can't remove a terrain once you upload it without a huge amount of work.
I forgot, the TE is named ahedit.exe in your game folder.