Well - first & most important, don't rush things. Let the relationship grow and unfold as it will.
Being the new person in it's important to remember that the couple has history, patterns etc they have developed in their time together and integrating you into those patterns etc will take some time. Potentially as much time as it took them to get to the point they are at.
This reality is what causes a ton of problems between new V's and triads (other variations). In the beginning there's a lot of energy and emotion and you hunger for time together. But there's still certain logistics that have to be acknowledged and figured out how to change or work around and despite all the desire and best intentions - it takes time. And focus.
As for having some alone time with each of them - that's also something you have to grow into. Until everyone has a strong level of trust & understanding of each other, requests for 'alone' time can be interpreted the wrong way ! Be careful with that. It's easy for one or the other to either feel left out, or undesired, if it isn't approached carefully. Best when possible to let those opportunities just arise as they will because of circumstances. Once it has happened a few times the comfort level should grow.
It may also be easier to start with the two gals going for some "girl time" - not necessarily sexual. Most guys can grasp that and not feel left out.
In any event, being together as a 3some is important so be happy that model is available to you. You'll find lots of folks that wish they had that and don't and will have to work really hard to ever get there. You're in a good place. Have patience - all will evolve.