It seems we confuse quantity for quality in relationships, most specifically dating/courtship.
"How can I throw away a 3 year relationship?" is used by people who can't accept that a failing on their partner's part is a red flag not to go into marriage.
It's the best red flag.

The point of courtship, or at least serious dating, is to know your partner better, experience life at a stretch with them, adapt to your differences.
Of course, you won't adapt fully, but you'll be better for it.
If you've dated someone for 2,3 years, and all of a sudden, they

do something you consider a red flag, it simply means they were pretending all this time, and drawing out the dating period exposed them for who they really were.
You should be happy throwing away the relationship, because if you'd rushed into marriage after a year, you'd

experience that red flag after 1,2 years of marriage, only this time it wouldn't be a red flag, but a deep gash on your marriage.
Divorce is messy, very messy. Especially when there are kids involved.
So, if you've been dating a man for 3,4 years, and on year 4, he beats you.

Take it that he's a wife beater.
End your relationship, don't try to defend the years spent. It was necessary that you spent those years, to avoid decades of punching bag status.