For the men, it technically happened in 1977 where there were 3 new champions (Vilas, Gerulaitis and Tanner) although it depends how you count the AOs (there were 2 that year). 2003 as well, as already mentioned. For the women, it happened in 1978 (O'Neil, Navratilova and Ruzici), 2004 (Sharapova, Myskina and Kuznetsova) and 2011 (Kvitova, Stosur and Na). Quite a few years on both tours with 2 new champions.
It's not all that surprising given how inconsistent the top players on the women's tour are at the moment. In the last 6 years (2013-18), Serena (8), Kerber (3) and Muguruza (2) are the only players who have managed to win multiple slams and they only won 13/24 slams. Compare that to the men, where you have Djokovic (9), Nadal (6) Wawrinka (3) Federer (3) and Murray (2), who have won 23/24 slams