I’m planning on making a 3D game but using the 2d sprites as enemy’s and pickups. Would I be able to make the environment and weapons in 3D? 

no I dont think so

I read the rules again and it mentioned that you can make 3D games as long as you use the art kit. I also saw Adam Pype do that for his game so I think it should be fine.

The whole point of the jam is to only use the art assets provided.

I suppose you can make everything in 3d as long as you are using the tileset assets.