Do you stay out of your friends realtionship or do you get all up in her kool-aid?

My best friend Martha is going out with my neighbor Rick who i've known for a long time they are perfect together but when i go out with Martha, Rick sometimes tags a long and i feel like the third wheel. When i told Martha that i felt awkward with them she just told me it will take time to get used to. But i don't want to ruin their relationship by complaining, but even if i don't get involved with them i feel like i'm gonna cause their break up and i'm gonna be the one getting hurt. Because if i stop hanging out with them they might think their relationship is going to end their friendship with me. And if i keep hanging out with them they might think of me as a third wheel. And i feel i'll be the cause of everything. What should i do?