The story was either in the “first time” or “old/young (mature)” category. It involves a girl and her 2 teen friends who visit her grandpa I think. One of the girls is a virgin and wants to learn about sex and the grandpa helps her out. I think he ends up fucking all 3 girls including his granddaughter. I also think he had a pool at his place.
Source: reddit.com/r/eroticliterature/comments/7s9vyh/searching_for_a_first_timeoldyoung_story_i_read
https://www.literotica.com/s/sex-lessons-from-grandpa