How acceptable an age gap is depends a lot on how old each person is. Typically for people who are under 18, age gaps are not okay. If both people are adults though (over 18), then I don't think age matters as much and an age gap can be as large or small as the individual person is comfortable with.
Even if two people are not the same age, if they share similar hobbies, interests, values, and beliefs then the relationship can work. In my relationship, there is a 6 year age gap between us but we're both adults and we have a lot in common so it's not a big deal for us.
I answer differently based on the age of the person asking the question. Like an 18 year old and a 13 year old is definitely a no, but a 35 and 30 doesn't matter. If you're under 18, I think 3 years is a little bit too large. To me age matters less compared to life experience.
3 years isn't that much, especially when you're both adults. 18 and 20 doesn't seem like a big gap to me. There's not a huge difference in maturity or experience. My current dating range is from 20 to 30.
Once you reach a certain age, it doesn't really matter. 18 and 20 are fine. For years I was in love with a guy 11 years older than me. Right now I am interested in someone the same age. For me, I prefer guys my age or older. However, if I met a guy a couple years younger that I connected well enough with, I'd date him. I've never been in the situation where I was attracted to a younger guy, but I don't think I could do more than 3 years.
A 3 year age gap is nothing. My limit is a 10 - 15 year ages gap. Yup, panther right here. Rawr! Lmfao! :) For younger guys, no younger than 20 for me. Bruce Willis is 61 and his wife is 37, that's a 24 year age gap.