i saw nline that there is a mod that lets you play ultimate doom as truly 3d.
does anyone know were can i find it?

What do you mean with truly 3d?

Are you talking about advanced source ports? GZDoom, Risen3D or Doomsday have some features that could be considered true 3d.

There is a mod that recreates the first episode in the Doom 3 engine. That's definitely 3d.

There is also a WIP mod that recreates parts in the Unreal Engine. I remember seeing a video that showcases one map – frankly it's horrible.

Some people try (or have tried) to recreate some of the levels with Snapmap for Doom 4. Obviously this is limited by the power of Snapmap that lakes the ability to create custom level geometry.