I can understand your pain, being taken advantage of by friends, making you a third wheel thinking you don't know it. Just stop hanging out with them. Or say something came up last minute at times. When you 3 are hanging out do they interact with you the same as they do with each other? Try hanging out with other friends. Even if you work together does not mean you need to hang out outside as well. Maybe you are worried they will think why you are upset and avoiding them, whatever don't let them take advantage of you by using you to get together.
