First off, there are no shortcuts. Everything comes at a cost. If you absolutely have to build a bike and don't want to gain any skill, maybe bamboo is the way to go. Lots of info about that. Or maybe we'll be reading about you and your 3d printed bike sometime in the future.

people print parts of bikes. Some are doing it just because they can, a few are doing a nice job of using the properties of the technique. Parts that don't have a thick section are probably not good candidates for printing. Just about the only thing I liked about the carbon-3d printed hybrid bike was how small the envelope was for the print, less than a 1 foot cube. Those bikes have carbon tubes glued into the lugs. Buying carbon tubes is an entirely different subject.

You could weld to these parts, but if you were good enough at welding to do that, you could certainly build a frame without any 3d printed parts