Treat it as you would a monogamous date or getting together with a couple of friends for the evening. Be yourself. Forget about everyone's sexual identities for the time being. There's time enough for that once you've gotten to know each other.
If you're at a loss as for what to do maybe take along a favorite movie or music, a game & any questions you might have.
In other words expect nothing but a good time & you're bound to have the good time. Anything extra is just icing on the cake.
There are as many ways to do polyamory as there are people practicing it!