Is Terraria already 3D?

No. The game's code is two dimensional with parallax to give the illusion of a three dimensional world. It's 2.5D at most.
 
Technically 2d but it is sorta a 2.5, think of it like smashing a glass panel straight through our world and smashing another in to restrict you to a 2d-ish plane. In other words, you are in a 3d world in a way, but you're heavily restricted via both game mechanics and how it would be messed up if Terraria was 3d. Another way to theorize is that you're some sort of omniscient watcher or whatever and are looking at the world in the way a guy looks at an ant farm.
 
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