The original
BSG (24 episodes, 1978 to 1979 season) never ran on NBC. It was on
ABC. The ratings were actually very good, but the show was too expensive to keep on the air. (As with every other SF TV show on American TV prior to
B5 the producers grossly underestimated their costs and quickly ran over budget.)
Galactica 1980 (10 episodes) was basically a new show with only two original cast members. The action moved to present day Earth to save on production costs. The ratings were deservedly awful and the show was cancelled. By ABC.
I think the 2003 mini-series ran as a "special encore presentation" or some such on NBC, replacing either a show that got canned or a live event that was postponed, but they never aired any episodes of any version of the series. The encore was probably as much a promo for the Sci-Fi Channel series as it was anything else.
It makes little sense to compare only the debut episodes for each season. True, the rating for the S2 premier was a 2.6. So what? The next week was a 2.0 and after that the show averaged about a 1.85 for the rest of the year, hitting a low of 1.7.
In other words, the ratings for S3 picked up just about where they'd left off at end of S2. There was no "big drop" from S2 to S3, that had already happened, early in S2. (Probably a lot of new viewers were drawn to the series by the publicity and either didn't like what they saw or they were put off by coming in late to a developing story and decided to check the show out on DVD. That's what I plan to do, actually. I'll probably start watching the show "live" sometime in S4.)
Regards,
Joe