Either on DVD, ABC's website, or Disney+ (https://popculture.com/streaming/news/disney-plus-adds-abc-sitcom-8-simple-rules-fans-have-some-issues/)? At least, that's according (https://tvtropes.org/pmwiki/pmwiki.php/Trivia/EightSimpleRules) to TV Tropes (https://tvtropes.org/pmwiki/pmwiki.php/KeepCirculatingTheTapes/LiveActionTV). Is it because Disney just believes that there's a general lack of interest in the third (https://popculture.com/streaming/news/disney-plus-adds-abc-sitcom-8-simple-rules-fans-have-some-issues/) and final season, since it's the only one that doesn't have John Ritter's participation what so ever?

I have no clue.

It’s available in great quality here. https://archive.org/details/8-simple-rules-season-3