I'm a little confused by what you're trying to achieve. if you want 3D HUD, parent it to your 3D UI camera. If you want in-game UI objects such as signposts that you can walk up to and click on, just create your widgets as a part of your world. Make sure each object has its own panel (ie: if you have a signpost with some text on it, make sure that the signpost has a UIPanel on it).