Regarding 1979-1981, I always thought of Austin being the 2nd best player overall during that period, behind Evert and ahead of Navratilova. This was how I traditionally ranked the 3 players during that period:
1979 - Navratilova > Austin > Evert (Navratilova is clearly top, and I have to give the nod to Austin over Evert)
1980 - Evert > Austin > Navratilova (that is definitely the most simple and clear cut year IMO - for me personally the the overriding memory was Goolagong's Wimbledon title win though)
1981 - Evert > Austin > Navratilova (that is definitely the most difficult year to rank the 3 players IMO - Mandlikova was also of course a very big factor - 1981 was certainly a very exciting year at the top !)
So based on that Evert is clearly top of the pile.
However as I said previously I think that there is a good case all things considered to give the nod to Austin ahead of Evert in 1981. If you do, then she's better than both Evert and Navratilova in 2 out of those 3 years, and there is a case for her being the best overall from 1979-1981.
Evert did win 4 majors during those 3 years compared to Austin and Navratilova winning 2 each. I will say though that the RG was clearly a lesser major compared to Wimbledon and the US Open during that period, and also had a noticeably lesser status on the women's side compared to the men's. The facts that men's tennis had considerably more depth and much more of an established clay court culture than women's were big contributing factors there - in women's tennis relatively few players looked strong / comfortable on clay for professional standards. Evert's big advantages when it comes to majors during that period are not the total number that she won, but the fact that she was the only one of the 3 players to win both Wimbledon and the US Open, and the fact that she reached 5 finals out of 6 (plus a semi-final during the lone exception at the 1981 US Open) at those 2 events, during that period.
Indoor tennis was clearly very important and very prominent, and also women's tennis was very US centric (far more so than men's tennis for obvious reasons). Austin overall had the best record during that period at the big indoor events, which is a big plus point in her favour. Overall the gulf between the majors (especially none Wimbledon majors so Austin not winning there is a negative point in her favour of course), and the big indoor events in the US wasn't that wide at all, and I agree that you could definitely argue that those top indoor events actually rank ahead of RG (and the Australian Open) for a period.
The head to heads between the 3 players during that period:
Austin 9-4 Evert
Austin 11-11 Navratilova (I think - I could easily be wrong there) - Navratilova beat Austin in a Wimbledon semi-final and Avon Championships final, while Austin beat her in a US Open final and semi-final, and in Avon and Toyota Championships finals, so I'd say that Austin had the upper hand in their biggest matches.
Navratilova 10-6 Evert
So Evert having losing records against both Austin and Navratilova then could count against her, while Austin having a clear winning record against Evert, plus the edge over Navratilova given her upper hand
in their biggest matches, is another plus point in her favour.
In terms of overall titles won during those 3 years (again I might be wrong with these numbers):
Navratilova - 31
Austin - 26
Evert - 25
I'm pretty sure that Navratilova did enter noticeably more tournaments overall than both Evert and Austin during that period, and played 50-60 matches more than either of them. In terms of winning percentages during that period, according to tennis abstract (so these numbers might not be fully accurate), it's very close:
Evert 234-32 (88.0%)
Austin 230-32 (87.8%)
Navratilova 278-42 (86.9%) - Of course to repeat Navratilova did play noticeably more matches than either Evert or Austin from 1979-1981, which is a major caveat here.
I stress I'm certainly not saying that Austin was way better than Evert and Navratilova from 1979-1981 or anything. I'm just saying that it is not at all unreasonable to make a case that she could be considered to have an edge (even if a small one) over them both during that window. Maybe I'm just trying to be a contrarian here ! If I'd made these points during my early days on this forum, when then there was more discussion of women's tennis in general, but also more 'strong' disagreements (to put it mildly - those Graf-Seles debates in-particular were quite something), maybe I would have incurred the wrath of some of the Evert and Navratilova fans around !