I tried finding it in my favorites with the tags I assumed it would have and couldnt find it so I’m pretty sure it was removed from nhentai during one of the many purges.
It was a full color doujin with the Mc being a skinny guy with glasses and he had sex with both his classmates a dark haired petite girl and a busty blonde girl. I think there were three or more installments in the series which eventually lead to the threesome with both the girls.
Blonde girl may have had a fang if I remember correctly.