3d Camera TRANSITIONS in AE. No presets? Another method??

Adding to Rick's comments, there's a script I've had forever called 3D Precompose from Video Copilot: https://www.videocopilot.net/blog/2014/08/free-script-3d-pre-compose-tutorial/

This script will create precomps with all the attributes linked to the original.  You could use this to duplicate your comp, which would allow you to make adjustments and have those changes mirrored in the other comps with the alternate cameras. That way when you come to fade between comps from a main comp, it shouldn't be as much pain to make adjustments.

You can move cameras, add multiple cameras in the same comp, and cut between them, but you cannot fade between cameras in the same comp. It is not possible and would be a terrible workflow. 

You can animate opacity to fade layers in and out of a shot, but you should not be putting more than one shot in a comp. AE is not a video editing app. Create your shots, render them, then edit them in an NLE like Premiere Pro.

If you have some kind of a transition that you cannot create in any other way, you can put two or maybe three compositions in a single new comp and use After Effects to create transitions between the nested comps. If you collapse transformations, any lights in the Main comp and the Main Comp Camera will control how the 3D layers in nested comps behave. If there is a camera in your nested comps (pre-comps) and there is a camera in the main comp, and you collapse transformations, all of the Camera moves, and the lights in the nested comps will be ignored. This is more of a blessing than a curse.  

I don't know if that answered any of your questions. It would help if we had a better description of your design goals and maybe some screenshots.