Schottenstein on the gemara, footnote 50 in the Hebrew edition, write:
"התנא שינה כאן מלשון הקודש שפתח בה, ואמר "תלתא" בארמית, לפי שכך שמע מרבו, וחייב אדם לומר בלשון רבו (1סידור יעב"ץ עמוד לו.)."
Translation: "The tanna moved from the Holy Language he opened with, and said "telata" in Aramaic, because that's how he heard from his rabbi, and a man has to repeat in the manner his rabbi said it (Siddur Yaavatz pg. 36a)."
I suppose this means that either the tanna heard the first portion of what he said from a different rabbi, or it's the same rabbi who happened to teach one portion in Hebrew and one in Aramaic (less likely).
1 Evidently Schottenstein used a different edition (or it's a mistake).