Just had a dig through the MAL comments section for the first one:
Definately not a sequel since some characters who died on previous seasons appear and also not a prequel either since all the team is together. As others previously mentioned is more of a side story, set in the same world but not connected to the main story line.
Who bets characters or events introduced in these “side stories” pop up. Always happens when people say but it’s a side story (Free and TYM springs to mind yes it’s side story OVAs but fills in blanks in s3)
It was caught up by the TBS deal with Anime on Demand UK so it never got proper streaming treatment. Seasons 1 & 2 did make their way over to Animax UK before they expired. Sentai Filmworks only have US & Canada rights.