3rd party isn't a strike and won't affect your standing with YouTube. Typically, it means that you used footage or audio that wasn't yours, was claimed by the person/ company who created it, and they can now track it and/ or claim any money from ads run on it.
Unless you were hoping to monetize the video and make millions from it, don't worry too much about the 3rd party claim.
…that is, unless everything was, in fact, created by you in which case you should fight it.