TT #263 · Parashas Reeh · Fifth Aliyah · Passuk 14:28
At the end of three years you shall bring forth all the tithe of your produce in that year and you shall leave it within your gates.
Torah Temimah points to Sifrei.
English: "At the end of three years" — One might have thought from Rosh Hashanah of the third year; therefore the verse says "at the end." One might have thought on Rosh Hashanah of the fourth year; therefore the verse says "when you have finished tithing." One might have thought even at Chanukah; therefore I reason: it is said here "at the end" and it is said there "at the end of seven years, in the appointed time of the year of shemittah, on the festival of Sukkos"—just as the "end" spoken of there is at the festival, so the "end" spoken of here is at the festival.
Derash on the verse (Sifrei) — the time of bi’ur is the festival after the third year.
The removal of the tithes of the third year is performed at the festival (Pesach) that follows the end of that year.