3D Transform was a filter introduced in
PS 5.5 to allow you to manipulate objects in 3 dimensions (hence the name). However, it obviously wasn't very popular, because from
PS CS onwards it wasn't included as part of the default Photoshop installation (it was hidden away as an optional extra on the Photoshop install CD, in the
Goodies folder). I've never used it myself, so I can't tell you much more than that.