I believe your boyfriend is wrong on ignoring how much this affects you and not offering you more emotional support. If he would actually discuss this with you instead of just nodding his head, you'd probably be able to get over this super quickly. I suppose you are all pretty young and at your age the concept of loyalty is still developing, which could explain his female friends' nasty comments. You do good in ignoring them, his ex is an ex for a reason and he has been with you for three years and this requires no other explanation
Those girls might be slow maturing and imagining that 3 years later their friend would be and hang out like he used to in high school. Fortunately neither you and him seem to have this problem and are building up a serious relationship that the eternal (and maybe a bit jealous too) high school girls can't have. Maybe when they'll work a bit more on their personality they will
I honestly think that they represent a connection to an important time of his life that he doesn't want to lose completely. Also he probably still needs to learn to communicate these things to you, but he has already started to see them less than he used to and more importantly, those girls represent his past, who he was, while you represent his future and who he is going to be. Enjoy your beautiful relationship and don't worry about two immature girls who still lack quite a bit of personality.