Welcome to the forums. You say you've never been in a relationship, so I assume you mean a romantic relationship. And you're considering being a third in a polyamorus marriage? My gut instinct is to say "NO! Absolutely not! This is a bad idea. You need to discover what is important to you in a traditional relationship first before jumping into the crazy polyamorous waters."
But really, who am I to say this wouldn't work? People are all different. It's possible you and this couple would develop a beautiful triad and live together in blissful harmomy for decades.
One thing that's for sure: Don't make things tougher than they need to be by pushing them - by demanding more of their time than they feel like giving at the moment. You guys will HAVE to communicate very honestly and openly about everyone's intentions to prevent any hurt feelings. Let them come to you. It's OK to voice your wants, but be receptive to the fact that they are inviting you into their lives. If you push for too much, too soon, you'll lose them both. Err on the side of seeing them less rather than more. Enjoy the good times, and try not to hang on to expectations of the future.