How would I put a video on a layer of a 3d object (3ds file
You don't. It's as simple as that. You're operating on completely wrong assumptions. AE has no access to the 3D data at all. It's the same limitation as with any 3D-ish effect in AE - they can only respond to the camera and to some degree 3D lights, but you cannot chnage textures or any of that on sa PS Live 3D file. Therefore your options very much only extend to placing your video as a 3D layer and matching the alignment or uinsg distortion effects like Corner Pin to pin the video to the layer as you would with conventional tracking. Another method is presented by using UV pass information, which is demonstrated here and explained here, but it requires you to use plug-ins and understand the workflow.
Unless you have full access to a 3D app supports exporting camera data to AE you're not going to have very much luck attaching video to your 3D TV if it's moving very much. If it's nearly static you can use corner pinning, scale and mask a 3D layer, of track the screen in Mocha and use the Mocha to AE tools to make it work well. The move you are describing is almost impossible without 3D camera info from the 3D app.
Any really big moves will work much better if you map the video to the TV in the 3D app. It's loads easier to setup. I've never tried texture mapping video to a 3D object in Photoshop and I don't know if that's even possible. Maybe someone else could chime in.
Thank you both for those comments. I understand a little bit more now that what I ask isn't really that easy. I see that effect everywhere so I thought it was moderately easy to do with after effects.
Rather than approaching it from the questions above, can we do a specific example below? I think a graphic will better illustrate to you what I am trying to do.
Below I have a flat screen 3D TV composition from Photoshop. On the face of the TV you see a video that I am trying to show on the screen of that TV.
How do I line up the corners of the video and also make that video play on the front of the TV? The TV will stay in one position and I will pan a after effects camera around the front of it and towards it as I learn more about how to do this.
Thank you both again for your help.
If the TV is relatively stable, that is to say, isn't moving around a lot, the easiest way would be to apply Effects>Perspective>Corner Pin to the video layer.
There are lots of options, but get started with Corner Pin and a TV that's not moving.
If the TV is moving around a bunch you'll have to learn about motion tracking and pre-composing.
Thank you so much Rick. But when I go to effect>perspective, there is no corner pin option. Is that a separate plug-in for purchase?
My mistake. Try looking under the distort category. You could also just search for corner pin in the effects and presets panel.
That really helped me a lot Rick - thank you!