A firkin is a certain measure; as urn, amphora, and the like. Metron is the Greek for measure: whence metretæ1. Two or three, is not to be taken to mean some holding two, others three, but the same vessels holding two or three.
Jesus saith unto them, Fill the waterpots with water. And they filled them up to the brim.
Reproduced in full. Translation: Oxford 1841 (Newman), public domain (Public domain).
Also cites: Tr. ix. c. 7