Anyone who has an interest in the property other than the buyer and the seller on the sales contract can be a 3rd party.
If it's an estate, the buyer, the executor of the estate, and the third party may be all the children.
In a short sale, the buyer, the seller, and the seller's mortgage company is the 3rd party.
In a divorce situation, the buyer, the seller, and the 3rd party could be an ex that is still on the deed.
Many scenarios exist..