Reference for Sanhedrin 1:2
מַכּוֹת, בִּשְׁלֹשָׁה. מִשּׁוּם רַבִּי יִשְׁמָעֵאל אָמְרוּ, בְּעֶשְׂרִים וּשְׁלֹשָׁה. עִבּוּר הַחֹדֶשׁ, בִּשְׁלֹשָׁה. עִבּוּר הַשָּׁנָה, בִּשְׁלֹשָׁה, דִּבְרֵי רַבִּי מֵאִיר. רַבָּן שִׁמְעוֹן בֶּן גַּמְלִיאֵל אוֹמֵר, בִּשְׁלֹשָׁה מַתְחִילִין, וּבַחֲמִשָּׁה נוֹשְׂאִין וְנוֹתְנִין, וְגוֹמְרִין בְּשִׁבְעָה. וְאִם גָּמְרוּ בִשְׁלֹשָׁה, מְעֻבֶּרֶת:
(Adjudications of) [forty] stripes (are presided over) by three, [it being written (Deuteronomy 25:1): "And they shall draw near to judgment, and they (two judges) shall judge them," and since beth-din cannot be evenly balanced, a judge is added, making three.] R. Yishmael says: "Twenty-three. [He derives it by identity: "wicked"-"wicked," it being written here (in respect to stripes) (Deuteronomy 25:1): "And they shall incriminate the wicked one," and, elsewhere, (Numbers 35:31): "…who is a wicked one (condemned) to die." Just as there, twenty-three, so here, twenty-three.] Intercalation of the month [i.e., sanctification of the new moon (Since "intercalation of the year" follows, "intercalation of the month" is taught.)] (is presided over) by three. Intercalation of the year (is presided over) by three. These are the words of R. Meir. R. Shimon b. Gamliel says: They begin with three, deliberate with five, and end with seven. [They begin with three to see if it is necessary to seat a beth-din for this. If one of the three says that beth-din must sit and see if it is necessary to intercalate the year because of the season, the spring, or the fruits, and two say that it is not necessary, that there is no doubt here, that of a certainty, intercalation is not called for, the one is in the minority and he is overruled. If two say to sit, and one, not to sit, the two are followed, and two more are added to deliberate the matter, so that now there are five. If two say that it is necessary to intercalate, and three, that it is not, the two are in the minority and they are overruled. If two say that it is not necessary, and three, that it is, the majority is followed and another two are added, making seven, who intercalate the year. The Gemara explains that these "three, five, and seven" correspond to the priestly blessing, there being three words in the first verse, five in the second, and seven in the last. The halachah is in accordance with R. Shimon b. Gamliel.] And if they ended with three, it is (nevertheless) intercalated.