Well to make it simple - I'd probably just email or ask them in person if they are polyamorous or in an open relationship, and if they said yes, I'd ask if one or both of them would be interested in dating me (or ask if either of them would like to go out with me). As you may find out the answer is yes or no, or only one of them is interested in you if they are poly, you won't really know your options until somebody sucks it up and says something.
I'm not big on mysteries or wondering about things, it makes too much work for my mind. I'd say life is too short to beat around the bush but I guess at your age you have plenty of time to do that if you want
I just imagine knowing is better than not knowing!
And there are a lot of tags on this forum marked *triads* which you should read if you haven't - and I've never been in one but from my inexperienced opinion I still think the advice to go slow is good, as the dynamic is so different from one on one dating.