Sloots gonna sloot. Just kidding, sucks to be you. I could possibly deal with a girl who has had sex with 20 guys but being a sorority slut (non-derogatory, can't think of any other term)? **** that ****, your girl was getting tossed around dawg. I can relate to girls who have causal sex because hey we're humans with sexual desires, but a girl who gets passed around in clubs/colleges for men/boys? Nah, having dealt with girls like that I just can't respect them. Some people might say that the person that's receiving is the same as the one giving but that's not true, I mean we all see value in having our garbage taken or streets cleaned, does that stop us looking down on those who clean our street or take our garbage? Nope.
Anyway, this is all how I feel. It's really up to you to decide whether this relationship is worth continuing. She's been in college for 4 years, that's around 5 guys per year, does that make it less uncomfortable for you? However, you can't really blame her for anything. She did not lie to you or mislead you. You did not ask so don't put anything on her. It's on you and your emotions, if you're not comfortable that's okay, sexual compatibility is important in relationships and you don't seem to have the same mindset. However, 3 year is a long time and you're planning on getting married so you might have a bias towards sticking with the relationship. You might want to see a therapist and discuss your feelings, possibly couple therapy after that individual therapy.
Next time, ask your perspective partner, there's nothing wrong with asking. If she states a number you're uncomfortable with then don't get involved. She's not going to be the only girl that's compatible for you, there are 100 Million+ women in the USA and I'm sure millions that have very low sexual partners.