New to 3D. Where Do I Begin?

yes they are still available but its a dead end

if the op is a Windows user and wants to spend extra $ for Stager then you could argue Dimension is still worth learning because its the same ui as Stager [Stager is just Dimension with Substance bolted into it]

Aero is just a demo and nothing more so yes have fun with it

to use Fuse you have to hack the creative cloud app [most schools don't allow cc to be installed so may still be teaching it] or you can still install the old Fuse for free off Steam... and yes the new Adobe models work in the old Steam one because they are the same thing under the hood but your average users doesn't know how to hack and Adobe isn't interested one way or the other

Dimension, Aero and Fuse are finished and Photoshop no longer has 3D so the honest answer is to move away from Adobe to learn

Blender is free and not going anywhere so its well worth learning... SketchUp is also very good and imo less of a curve for new comers

as has been stated it helps to understand what you want to get into and what hardware you have to use because Mac limits how much you can do in 3D to Unity and Illustrator but Windows opens up Iclone

You might have an easier response if we know what you want to make with 3D.

Dimension is a bit limited for many things - (I'm a professional 3D modeler, animator using Cinema4D & Maya), but for someone just starting out Substance might be really overwhelming.  (I'm going to start my Substance journey soon!)

Dimension would be a great place to start thinking about manipulating elements in 3D space, consider lighting, application of textures, and rendering/compositing options.  You can graduate to Substance when you're ready.

Adobe Aero is a really fun application for your mobile device to play with AR space. "AeRo".  You can import models from anywhere to Aero (maybe from Dimension? I haven't tried yet)

There is no Photoshop Fuse anymore. (sad face) ... (very sad face).

I'm new to Adobe's 3D tools as well (not counting the original Adobe Dimensions, circa 2000).
I think it depends on your 3D goals. Are you wanting to create product concepts? Then Dimension is a good place to start. Since it is already part of the CC app family you can download and start learning. The downside is that if you need custom models you'll still need to learn a 3rd party app (such as Blender) to create your 3D assets. But the lighting and scene building is much easier in Dimension than in Blender.

I don't yet have access to Substance since it is separate from CC. It appears it will soon have a Modeler though, which is encouraging news.