DON'T BE "THE THIRD WHEEL!!!"
Believe it or not, there once was a time when I was not awesome. It was a long long long time ago, but it did happen. However, I eventually learned from my mistakes and rose like a phoenix from the ashes so that I may offer others the wisdom from my (very few) mistakes.
Why? Because I care.
During this Dark Age of my existence, I spent the majority of my time with one of my very best friends and his girlfriend. We ate together, cruised together, went to the movies together... we were practically inseparable. Only after he broke up with her did it occur to me how awkward our relationship was...
I was the tagalong. They always invited me, but that's irrelevant. What was really happening was that they were dating, and I was spectating.
The term "Third Wheel" originated from the term "Fifth Wheel," an object which, on any form of transportation, would be considered USELESS until a blowout occurred and a SPARE was required. (You can take that extended metaphor however you want.) Similarly, there are only two vehicle types that effectively utilize three wheels: the tricycle/big wheel design (which are for dumb kids who have no balance) and some foreign/experimental cars (which are for no one).
Don't do this to yourself. You're wasting your time. If you want to hang out with the guy, schedule some Guy Time. You're never gonna have 1-on-1 time with the girl, and you shouldn't want any, either (unless that's one of the main reason you keep hanging out with them, in which case, just cut yourself loose from them until you get your priorities straightened out).
Once, I scheduled a lunch-date with a lady-friend just to catch up and hang out. A couple hours before we were to meet, she asked if her boyfriend could come along. Sure, he and I were friends, BUT IT DIDN'T MATTER! She had turned a casual friend-date into a date-with-a-tagalong. I informed her of this and refused to meet her. And I never saw her again.
She acted hurt, but she knew what she had done. She snuck him into the event, because she wanted to be with him more than me. Never allow yourself to fall into this trap, because the couple will always prefer each other's company over you.
And honestly, can you blame them? They get to make out with each other.