TT #705 · Parashas Ki Seitzei · Seventh Aliyah · Passuk 25:07
And if the man does not desire to take his yevamah, then his yevamah shall go up to the gate to the elders and say: “My husband’s brother refuses to establish a name for his brother in Israel; he will not perform the duty of a husband’s brother for me.”
Torah Temimah points to Sifrei / Sanhedrin.
TT lemma: אֶל הַזְּקֵנִים.
אֶל הַזְּקֵנִים – זִקְנֵי שָׁנִים; וְאֵין בֵּית דִּין שָׁקוּל, מוֹסִיפִים עֲלֵיהֶם עוֹד אֶחָד – הֲרֵי כָּאן שְׁלֹשָׁה; מִכָּאן דְּמִצְוַת חֲלִיצָה בִּשְׁלֹשָׁה דַּיָּנִים.
English: “To the elders” — elders in years; and a court may not be even, therefore they add one more — behold three; from here that the mitzvah of chalitzah requires three judges.
On the daf / in context: The minimal court for chalitzah is three, corresponding to the smallest competent beit din.
Derash establishing the size of the chalitzah court.
Chalitzah requires a court of three judges (preferably five for greater publicity).