URGENT: my boyfriends bestfriend😫

okay guys so i met my boyfriends (of 3 years) bestfiend a few months ago and we clicked instantly.. i really like him but i dont know how to break up with my boyfriend so i can be with his bestfriend. his bestfriend told me he wants to be with me too and we have kissed but havent gone any further because i know that if ny boyfriend finds out itll already hurt him enough. i have tried to be lowkey about it and suggest a threesome with someone we were both "confident" around and at first he said his bestfriends name but then he said he doesnt want to share me with anyone else. do i break up with him and tell him the truth or break up with him and make sure he never finds out about me and his bestfriend?