from Wiktionary, Creative Commons Attribution/Share-Alike License
- adj. third.
Sorry, no etymologies found.
Certainly, the force of analogy from the usual -th ending has had an impact on the irregular first, second, and third, so we do find oneth and threeth as well.
Well, you already have 'fourth', so all you need is to rationalise the series with 'threeth' and 'fiveth' and you've more or less done everyone a favour!
"There, that's the _threeth_ time you've said so, Fel Allen."