What are the implications of the word "Az Yavdil Moshe ... "?
Rashi (citing Makos, 10a): It implies that after carefully contemplating 1 separating the three towns of refuge in Eiver ha'Yarden 2 Moshe now put his thoughts into action.
Ramban and Seforno: Having gathered the people to explain the Torah to them and having concluded the introduction to his explanation, 3 Moshe now designated the three towns in Eiver ha'Yarden (to demonstrate the importance of keeping the Mitzvos - Seforno) by fulfilling even just a segment of a Mitzvas Asei. 4
Oznayim la'Torah: Having been told that he would not cross the Yarden - and would therefore not be available to teach them the Halachos of Arei Miklat when they fell due 5 - he took the opporunity of teaching them the Halachos. 6 This is bearing in mind that the main function of Mishneh Torah was to teach the detailed Mitzvos of Torah she'be'al Peh, and what better way was there to do so than by on-hands demonstration.
See Sifsei Chachamim.
Rashi: Even though they would not take effect until the three in Eretz Yisrael were designated - because he said 'If a Mitzvah comes to hand, let me fulfill it!'.
Ramban: Incorporating his rebukes, his warning against Avodah Zarah and the Mitzvah of Yichud Hashem, and a warning to keep the Mitzvos in order that Hashem does good to them.
Refer to 4:41:1:1**
When all six towns had been designated.
Oznayim la'Torah: For example: The size town that was eligible to be chosen, that it should not be surrounded by a wall and that it should contain water markets that supply food.
Why does the Torah insert Moshe's designation of the Arei Miklat in the middle of his speech?
Rashbam: It is in anticipation of what he will later state in Parshas Shoftim, 19:2 & 8 "Shalosh Arim Tavdil lach ... ve'Im Yarchiv ... Veyasafta lach Od Shalosh Arim ... ", where he does not mention the three Arei Miklat that he had already designated here in Eiver ha'Yarden.
Why does the Torah mention that the three Arei Miklat that Moshe designated were Mizr'chh Shemesh" - in the east?
Rashi: Refer to Bereishis, 4:16:1:1.
Makos, 10a; Hakadosh-Baruch-Hu said to Moshe 'Cause the sun to shine - Give life! 1 - to the murderers!'
See also Oznayuim la'Torah. citing Rashi.
