I want to tell my bf im bi Watch
I'm very certain that he'll react well to it, so don't worry and talk to him. He'll understand. If he doesn't then you deserve to be with someone who'll love you nonetheless! Have courage and good luck!
If he dumps you after 3 years of being together then block him and move on. You will find someone that loves you for you, regardless of your sexuality.
The second thing to do is talk to your bf.
Regardless of him being Christian, he should hopefully love you for you otherwise you wouldn't still be together after such a long time.
As others have said, if your bf dumps you because you're bi, you'll be better off without him and you'll be able to find someone who genuinely does love you for you and everything about you, not just the parts of you that they like or agree with.
The sooner you talk to him about this, the sooner you can put it behind you, and move on with your lives together, either together or going your separate ways.