A third wheel is the awkward extra person on what was meant to be a date. If your kid says "I was the third wheel last night" they tagged along with a couple and felt left out. Not concerning. It is a long-standing English idiom, not new slang. Sometimes used self-deprecatingly when a teen is single and friends are pairing off. If your kid uses it constantly, ask them where they first heard it, not to police their speech but to understand which online community they are in.