I want to concat three videos together, but they have different (1) frame size (2) bitrate, and (3) framerates.
To avoid glitches, I am resampling each clip on a consistent basis.
1) What is a good basis - bitrate , frame size, frame rate - to resample all the files? I'm looking for best quality and don't mind bigger file size or longer times.
With files of arbitrary attributes you need to decide on a set of parameters to conform them all to. If you don't have any strict requirements then you can refer to the inputs to see what is most common among them.
In your case you have two inputs that are 720x1280, and two that are ~30 fps, they are all stereo, all 44100 audio sample rate. So those can be your common parameters. Ignore the input file bitrate: it is not important.
Then use a series of filters to make all inputs the same before feeding them to the concat filter. For examples using the ffmpeg command-line tool see:
As for "best quality" that depends on your encoder settings for whatever format(s) you need. See the encoding articles in the FFmpeg Wiki.
