There are plenty of ways to spice up your relationship that don't involve bringing others in. I'd consider doing those and seeing if they scratch that "something new" itch before attempting to make a totally closed relationship into something else. You two have had a basic understanding for 3.5 years. That's a significant amount of time and it's quite possible that your partner will be stunned by your desire. (Of course, there is always the possibility that they love it. But since you two guys don't apparently talk about this stuff with each other, I'm just going with the odds.)
Anyway, the danger with the threesome is that you could show (or your partner thinks you show) more interest in the new person than your own partner. If this happens, it can ruin the relationship no matter how many reassurances you to try to make.
Also, if you're a heterosexual couple, you need to be ready for the possibility that your girl will want to be fucked by other dudes. I'm gay, but I know that many straight guys have a difficult time with the idea. It may start out as bringing a second girl into the bedroom, but fair is fair, and your girl may eventually want some new dick. If you can't handle the thought, don't even begin to head down this path.