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), in that he fulfilled just a segment of a Mitzvas Asei. 4
See Sifsei Chachamim.
Rashi (Ibid.) and Ramban: 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 do good to them.
Why does the Torah insert Moshe's designation of the Arei Miklot in the middle of his speech?
Rashbam: It is in anticipation of what he is about to state in Devarim 19:2 & 8 "Shalosh Arim Tavdil lach ... ve'Im Yarchiv ... Veyasafta lach Od Shalosh Arim ... ".