Please advice my future marriage needs help

Ok ladies, so I tested possitive at my doc for chlamydia....I have been with my fiance for 3 years. He claims that he has never cheated, I started getting symtoms in June.... I am getting married to him in 3 weeks and I need advice. There is no way I could have had it for 3 years and am just now getting symtoms...right? This was my first std check and his too. Please help me ladies....did he 100% cheat or is there a slim chance he didnt?