What advice can you give about a third wheel?

the situation is this,


My girlfriend has a close friend that always hangs out with the two of us since her friend does not have many friends. Now, in a stat in fb, her friend posted, ';I hate being the third wheel, why can't you understand how I feel?';. what should we do with her? she's the close friend of my gf, and it's hard to leave her friend just like that? any advice? it would be appreciated.. :)What advice can you give about a third wheel?
i am the ';third wheel'; with my besty just make the friend feel welcome to hang with y'all and give her attention (not enough 2 make your gf think u like the other girl) like get a inside joke or something or a cool hand shake(i no it sounds cheesy but it actually entertains me alot lol) or one day you invite the friend out 1st so she dosent feel like a burdon on y'all going out. try to befriend her so she feels comfortable around her or just leave her home