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 !
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