My advise comes from my experience, which is brand new. I've been in a poly quad (2 long term couples) for just over 3 months and we're definitely still changing. We've been sexual from very early on. As with most advise, this is solving the problems that I've encountered, not necessarily those you will encounter.
Our typical pattern is that we're together for clothes on activities and mostly separate when clothes are off. We've done some separate clothed/casual activities but not yet anything resembling "dating" (i.e. the cliched movie-and-dinner). Separate activities don't feel threatening (currently), possibly because we haven't rushed them. We just haven't paid attention to it -- I suspect we will in the near future.
From my experience, I suggest heavy doses of reassurance (love, affection, words -- whatever their love language might be) for your existing partner -- NRE is awesome but it can be unsettling to see your love acting so much happier with them than with you, and lavishing that love on your existing partner is a reminder both to them *and* to you about how much you love each other. My own feeling is that when that NRE happiness spills over on me I'm sharing in her life and love even if not in the activities. It's awesome to see my love so happy, not so awesome when she's grumpy with me because she's missing him.
I also suggest everyone really considering the concept of "move at the speed of the slowest person". It's easy to get frustrated when you really want something that someone else finds challenging. It's particularly frustrating when they've stated either that they want it eventually or that they're willing to explore it and you don't see the moving towards what you want as fast as you want. It's also a learning experience -- even if someone says "I want X", as they get close to "X" they might have different ideas -- perhaps they don't actually want "X" or want to approach it differently. I've been on both sides of this issue.
Remember that when you respect the speed at which someone else wants to move, you're taking care of them and giving them a gift (both of these are things I want to do with those I love). If you take on observing their boundaries as your own goals not only does it make them more comfortable but also allows them to more freely move their boundaries rather than fighting to maintain them.
I suggest making a place where everyone can share their feelings and desires and be heard and no one feels attacked for where they are. It seems that "I'm not sure I want that" for something that you really want is harder to take than "I really want that" for something that you don't. Separate the sharing of the feelings from the solving of problems and making agreements -- and put some time in between them. Whenever something new comes up that you haven't talked about, remember everyone's feelings and work towards making them safe. Be honest with your feelings and remember that things can change -- both for you and for others. Respect where everyone is -- including yourself -- but don't get attached to it.
It's much, much nicer to be interrupted when you're in the middle of something (even making love) with someone checking in on "is it OK if..." than it is to deal later with "we weren't sure but...". This is true for both directions -- "we weren't sure, so we didn't" can be upsetting for you to hear when you're feeling compersion. It's also much nicer for yourself to *know* you've got the go ahead. Yes, even if you've talked about it before. Make space for someone to tell you "yes".
Do things to create joy and encourage love. And, yes, every polycule seems to be different, and it's a bit frustrating. GalaGirl spelled out the combinations -- is it any wonder that it's so much more complex than a duo?