What are the connotations of "Oz Yavdil Moshe ... "?
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 just a segment of a Mitzvas Asei. 4
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.
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 see fit to mention that the thre Arei Miklat that Moshe designated were in the east?
Rashi: Refer to Bereishis, 4:16:1:1.