When my ex-husand and I hung out with his gf, he would sit in between us and hold both our hands, the same the once the guy I was dating came to our house to watch a movie with us. For hand holding that seems to be...equitable. A short hello kiss and a bit longer goodbye kiss with the third person seemed to be fine with everybody in those situations, during early parts of the relationships. I suppose if I tended to french kiss people in public in general there would be more of a need for me to worry about if I was making anybody uncomfortable
At a coffee date I don't imagine I would show any more affection to my husband than a light brief touch on the hand or the shoulder, whether it was to get to know one of his partners or one of mine. At a party or other group event I would probably try to share the equal amount of physical affection with both my partners after a discussion about everyones comfort level, and he would do the same (exception being if one of my partners was at the event with their own date)
I do remember going to a cuddle party with my now ex while I was dating the man I'm married to now. My (then) husband was looking forward to meeting new people, and as my bf had asked me there, he assumed my bf was my date, I assumed my bf was my date, but my bf assumed my husband was my date, and I ended up being politely semi-ignored by both of them out of respect for the other. Luckily I ended up spending lots of quality time kissing some really great women which helped, but it has taught me to talk about things better ahead of time.
I find it different now that my husband is dating a woman who is married, that does not make me feel like I shouldn't hug or kiss him in front of her even if she is not currently cuddling with him in front of me, so I imagine I do tailor my affection levels in front of OSO's based on if they had spouses/serious relationships vs. being relatively single.