Might someone have herpes and have unprotected sex for 3 years before spreading it to your partner?

Yes indeed. This is very common. Studies show that in monogmous couples in which one has genital HSV2, who have unprotected vaginal sex 2-3 times a week, transmission occurs in about 5% of couples per year (1 in 20). It works out to about one transmission per 1,000 sexual exposures. This low rate assumes no sex during obvious outbreaks. For HSV1 from a partner's oral herpes, transmission is even rarer.