While Phil has already given you the best answers, I’ll sum up with my two cents.
If your end usage will be with mp3, then by all means save your Golden Records recordings as such.
If you plan to burn cds from your vinyl (or tape) recordings, it would be best to save them as WAV files. Format: PCM, and Attributes: 44.100kHz, 16bit, 172kb/sec.
You can set GR to burn a disc in the project wizard, however, for various reasons I often will want to work further with the files, usually in WavePad, before I am satisfied with the project quality and tracking. I then burn discs with Express Burn.
FLAC is a lossless compression; mp3 is a lossy compression. There is nothing to be gained by converting an mp3 file to FLAC. If your desired end result is a FLAC file, then start with WAV and convert that to FLAC. The NCH software designed to convert audio formats is Switch. Check the NCH Switch page to learn what file formats it supports; the version I use (4.22) does not convert to ALAC. It works very well converting WAV to FLAC, or mp3 too for that matter.