[sattoshi]

Troisieme is the third when counting. “Tiers” is the third when taking a fraction.

I can only speak to modern french though.

[davidivadavid]

That got me curious, and according to the French Wikipedia, he was originally called "Letiers" (with the same etymological explanation) before he renamed himself "Lethiers" and finally "Lethière". Seems to hold up.

[srdgh]

"Tiers" can mean "third" in a sense other than fractions. E.g. Le Tiers-Etat (the Third Estate in the Ancien Regime) and "un tiers" ("a third party").