From your wording, I'm taking it you are a male talking about your female partner? 3 years is quite a while in the "open world" and you're fortunate it's lasted this long, but don't keep on the rose tinted glasses and expect it to stay this good. If anything, she may have already had eyes on someone else for a while without you realizing it. Male or female, the only reason a partner would suggest an open relationship or polyamory is they already have their eyes on someone else but don't want to be viewed as a cheater and still have the emotional safety of dating you. Don't expect the relationship to go from open to closed again. Some doors, once open, are open for good. You are not the first or last to think it's a possibility. Unless the idea of being a cuckold is an active turn on for you, it's like a ticking time bomb.