Commentary for Menachot 196:11

The middle is preferable. And the explanation of the book [Rashi] is regarding that we read the Torah that we do not read any less than ten verses, and the middle reads four verses, being preferable. And regarding this he brings the baraita there, in the third chapter of Megilah (Megilah 21b) but it is difficult - because they bring also other baraitot, that affirm that the first and the last [alyiot] are preferable [as well]. Therefor it seems that here in our mishnah regarding the three that walk in the way, the bigger is on the middle, as it is said in the chapter "The appointed one said to them" (Yoma 37a) and in Masechet Megillah (Megillah 21b) it brings that since other things are preferable, it is preferable that he reads 4 verses.

From here we infer that the middle position is commendable - regarding three that read Torah on Mondays and Thursdays, and we say in Megillah 21b: "we do not read any less than ten verses. And Rabbi Yochana says: the middle [alyiah] is preferable, and that is four verses, and the others are three and three.