Welcome to our forum.
Just remember that as an "add-on" to an existing couple -- they might dub you a "secondary" partner -- you have rights and should stand up for them if they get violated.
Check out this link:
Would you mainly just be connecting with one of the two people, or would you more be connecting with both people? I get the impression that being "secondary" is okay with you because you don't want to have to commit 100%. Just make sure you stand up for your rights too.
Glad to have you with us.