Bonjour Paul !
The expression is une tierce personne as in a third party in English. It's usually used in a legal context for example.
I cannot really think of other cases when we use tierce, as tiers is the one you used when expressing a fraction, hence in most cases:
un tiers, deux tiers...
Un tiers du monde -> a third of the world
I hope that's helpful!
Bonne journée !