Why are you asking this question? I think you’re looking for people who don’t know you to back you up, which is silly. But in case that’s not true, here’s how you do it:

You arrange to meet the person privately and explain your feelings honestly and compassionately. And then you live with the consequences of your decision without being petty or weak.

It seems to me that someone who knows how to date someone for 2-3 years should know how to express their feelings to that person. If not, maybe the problem is more complicated than you think.