Is is bad to date someone that is 3 older than you?
I really like him and he likes me and I've been knowing him for a long long time
Depending on your age. If your in high-school or older then no. If you are in middle school or elementary school then yes!
No it's not bad?
If he's an adult and you're a minor, than no it's not ok. If you're both minors, you're too young for dating.
No it doesn't matter