A lot of people think threesomes will be easier, but that's not necessarily the case. What if you reeeally like the third person, but they like your boyfriend more? Then you'd be jealous of *both* of them! Plus, in a group situation you have to actually see your boyfriend be intimate with someone else, which can be very hard for some people. I <3 threesomes, but they definitely don't protect against the scary stuff.
It sounds like your boyfriend will still be poly and still need this no matter what, so why worry so much about what your personal relationship to poly will be? You may not really know for some time. Like, you may fall for someone really quickly but then eventually realize that you feel more mono and just want to be serious with the boyfriend after all. Or it may take you a long time to find a partner but when you do it could be someone you want to share your life with long term alongside the boyfriend. This stuff is way unpredictable.
I think it makes more sense to focus on whether you're getting what you need from your boyfriend -- enough love, sex, quality time together -- and if you are, then realizing that him being close to someone else won't take that away from you (except maybe some of the time, but not necessarily moreso than if he, say, joined a new sport or was taking a class). Even if you *are* mono (which is totally ok) you can still learn to appreciate the energy, friendship, and support that his other partners bring to him (and maybe even to you -- metamour friendships can be awesome). That seems like a better plan to me then worrying about whether or not things are even or trying to set up arrangements where jealousy won't be an issue.
Me, 30ish bi female, been doing solo poly for roughly 5 years. Gia, Clay, and Pike, my partners. Davis, ex/friend/"it's complicated." Eric, Gia's husband. Bee, Gia and Eric's toddler.