Many older doors have 5" backsets while modern doors use ~2.5" backsets. On top of that, many older door knobs used a slightly narrower shank, so new doorknobs won't fit into older latches. 

If you're not planning to replace the door entirely, then you may find it difficult to update the doorknobs without also buying new latches separately. Even if you can 5" latches which are compatible with the knobs you would like to use, they tend to be shockingly expensive. In my case, the only latches that would work were $50 each and the only real difference was the shank width. So rather than spending $300, I fired up the printer instead.

While I can't guarantee that they will fit your door latches, they should fit most modern basic home door knob brands, like Schlage, Weiser, and Home Depot's Defiant brand. Note: Kwikset uses a proprietary crescent shaped shank, so this cam will not work with those.

These cams go inside the door latch. The process for disassembling the latch will vary depending on brand and age. On my door latches, two steel tabs held a set of plates and a sprint together inside the latch cylinder. I found that the easiest way to replace the cam was to hold the plates in place while prying the tabs open. Then I spread the plates open just enough to swap the old cam with the new one, being careful to not decouple them inside the cylinder, before closing the plates back up and bending the tabs back into place.

Using a strong filament is highly recommended. PLA might work, but I would be concerned about its long-term durability under repeated load. The original cams that came out of my latches appear to be nylon or ABS, so using one of those would be ideal. PETG would likely be fine as well.