Looks like it was actually 1988..
News of the Weird reported in 1988 on the Japanese inventors? expo at which was introduced "six-day" underwear -- a cotton panel with an elastic band and three leg holes, so that the wearer could rotate one leg hole per day and then turn the garment over and go for three more days.
In February 1998, two Metairie, La., women obtained a patent for pantyhose with three legs so that the spare can be used when a run develops."