Ok, I'm not 100% sure what you mean by 3D backgrounds. As far as I'm concerned, 3D backgrounds in a custom level are done by a glitch of some sort. I remember there was a level called Tutorial: How to make 3D levels by thi766 just search it in the user-generated levels and it should come up. In the level you get the things to make extra layers or custom backgrounds.
NOTE: I heard rumors that there was a patch that fixed this glitch, so I'm not 100% sure if it is still there, or if it even works. Worth a shot though. Other than that, I don't know of anyway to make custom 3D backgrounds.
Unless of course, you make your level on the very 1st layer, then the other 2 layers can be used as a background and you can use materials to create it. I used to do that. Except I used to make a 2 layer level, with 1 layer dedicated to the background. You can make the materials super thin, so they don't even have to take up a full layer. Mess around with the R1, R2, L1, and L2 buttons to figure out the whole layers thing.
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Now, as for the other question: Maybe an emitter would work. You're gonna have to fool around with the tweaking on that to get it. Thats all I can really say to that one, I would have to run LBP again and try to remember everything. Thats just off the top of my head. The Emitter should work if you can tweak it right. Give it a shot.
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