How do you complete Mario 3d land W6-3?

I really don't understand what you're trying to say. If you mean "is SM3DL like Super Mario 64, Sunshine, Galaxy, Galaxy 2, etc.?", then yes. It's played in a 3D plane, but the levels are very...

most likely you must do some thing like beat the game

You must finish Worlds 1 through 8. Then go to World 1. Go to the very beginning of World 1. You'll see a pipe which you'll have to go through to get Special World 1!

get the grey takooni leaf in the special world and jump and press R1 or L1

the answer is yes but you get tankou suit more. so fire mario is very rare.