I don't have an Ender 3, but I believe my 2014 FDM printer is not as good as the Ender 3 so take this post with that in mind. For reference my FDM is a PrintrBot Simple Metal - a good little printer by 2014 standards. So what can you do with FDM? Servo mounting plates - absolutely. I've done several practical prints for things around the house on my FDM, mounting plates would be easy peasy. Bridges and buildings - yes! I'm attaching a couple of examples of what I've done. Mind you I've used blended media to enhance the mediocre output of the printer, but at least you can see what you can do...
This is two bedroom house modeled after a prototype in Louisiana. A lot of thought went in to making this work. For example the patio and carport supports - on my old printer I would not want to print them vertically, so those were printed horizontally. The roof (all of them) were printed on end in one piece. The external walls of the house were printed as if the house were just sitting regular on the print bed - the internal walls were printed upside down so that I didn't have to deal with "bridging" over the doorways. The window frames and doors were printed individually and so on. To mask the fact that this was 3d printed, I "paper printed" textures for the roof tiles and siding then glued the paper to the structure. Oh, and the floor was the same - hardwood floor printed on paper then glued to the 3d printed floor.
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This is some test fitting of a WPA overpass I'm constructing based on a prototype in my grandmother's home town. Deck and girders were printed upside down in one piece, the railings took a LOT of messing around to get the look I wanted but eventually I found the magic formula. The support structures and couplings to chain several components together were all FDM printed. The roadway was...printed on paper and glued. I personally love how that turned out. Since I do own a Photon I am going to do the railings on the Photon - it's just easier and less cleanup effort and better detail.
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For filaments - PLA all the way. It's easy to work with, perfect for the Ender 3, cheap, comes in a huge variety of colors, and unless you're planning on putting these things outside PLA is fine for indoor use. For Ender 3 specific questions I'd check out the YouTube channel "CHEP" - he talks about the Ender 3 vs Ender 3 Pro, upgrades, tips for slicer settings and works with Ender 3s almost exclusively. I enjoy his channel even though I don't own an Ender 3 - I've been thinking about getting one but I think I'm going to go Prusa Mk3 to spoil myself. My old FDM requires too much tinkering to get good prints so I'm ready for the Cadillac of FDMs.
Cheers -Mike