ffmpeg to convert and merge 3 videos to a single widescreen video
Quality-wise, it's better to use the original MOVs, but if the MP4-based stacking looks ok, that's the final arbiter.

ffmpeg -i left.mov -i centre.mov -i right.mov 
-filter_complex "[0:v:0][1:v:0][2:v:0]hstack=inputs=3"
-c:v libx264 -tune film -crf 16 -b:a 256k triple.mp4


When using the originals, it doesn't matter whether you do it in one step or two. For streamlining purposes, I would do it in two - then the two encodes aren't competing for memory or CPU.


The stack filters are somewhat more efficient. They require all inputs to have the same size in the dimension normal to the join direction i.e. same height for hstack. Overlay is a generic filter, so it doesn't. Also, stack filters will freeze on the first and last frame of inputs which start late or finish early.
In digital files, audio and video are independent i.e. they exist within the same file and are synced via timestamps but logically they are distinct entities. ffmpeg relies on -map options to select streams for inclusion in the output. In the absence of any -map option, it picks one audio - the "best" - from among all the inputs. See https://ffmpeg.org/ffmpeg.html#Stream-selection
[1:v:0] ... "means 'second input, first video track only'?"  --> Yes.
Looks fine. The suitability of the tune value depends on the material.