HELP!

****update****

I caught the son of a bitch. It was a dude madly in love with me trying to break us apart. Two of my close friends and I all cornered him and caught him in his lie. Teaches men not to mess with women. We will fuck your life over if you try. *************

I need advice, I don't know what to do or what to think anymore. I have been with my boyfriend for about 3 years now, although I have known him my whole life. Things have been pretty good, were getting close to being married, looking at rings, houses, etc. Its all pretty exciting until about a week ago I received some pretty alarming text messages from an anonymous person whom I have absolutely no clue who it is. My boyfriend and I are on this gaming chat called discord, we play online games together and sometimes talk on voice chat with out friends using that app. It has what are called servers, someone creates a server, you get invited and its like a chat room that only those invited can use to either talk in voice chat or text. This random person sent me screenshots of a conversation apparently with my boyfriend. the texts are pretty disgusting. my boyfriend (maybe him, I dont know) said stuff like "the only time she shuts up is when shes blowing me" and "I keep her around because it means i get to have good sex" .... heres my issue... I have absolutely no clue who sent me these, my boyfriend denied that it was him, he said it must be someone on the app that's trying to break us up, (theres this guy that likes me and wont leave me alone).. I guess I dont know what to think. ive been with him so long and hes never pressured me to blow him or have sex with him. in fact i haven't even given him that many blow jobs in the 3 years we have been together. I want to believe my boyfriend, but i go back and read the messages and they sound exactly like him. the way he types etc. everything sounds like him. he told me if i didn't trust him he would leave. so i told him id forget about the messages and move on. but i cant. i keep thinking about it, i keep wondering if its really him or not. i dont know what to do. Do i go with my gut feeling and leave him after 3 years? or do i trust him and try to forget about it? also, there is a few more things he said or that were said in the messages but i didn't want to go to far into detail.