Everyone has been or will be the third wheel sometime in your life, it's inevitable. Being the third wheel is not as bad as the movies make it out to be, they don't ignore you and they are not across the table making out in front of you. If they are, you might as well just excuse yourself from that date. There are a few rules to being an awesome third wheel for your friend.
1) Be nice/ Your friend likes them for a reason. Give that person a chance before you automatically don't like him or her.
2) Tell funny embarrassing stories about your friend that they won't be upset with you saying. That way you can break the ice with your friend's special someone and get a good laugh in the process.
3) Hangout with them a few times so you can get to know your friend's special someone, so that if you are unsure about that person you can get to know them more. Figure out if you should tell your friend if something is up.
4) Go on dates... YES, dates. Go out to dinner with them. Either invite them or y'all have something planned, but randomly dropping in is a big no-no.
5) Really get to know the guy or girl. I am actually snap chat best friends with my best friend's special someone. Kinda weird I know, but as my bestie says, "We are a package deal, you get us both for the price of one."
All in all, just make sure that whoever your friend is, make sure they are a good person. Make sure they are going to be good for your friend and treat them with respect. Don't be afraid to tell your friend if you think the person they are with is bad news. It's your job, as a friend, to look out for your friend and to help them. I have had arguments with my friend over whether this guy was good or bad news and it was not fun to argue with her. She eventually realized I was right and that two weeks of arguing was worth it. Boys and girls come and go but friends are forever so just always be there for your friend, in the third wheel dates and the hard times!