מִן־הַשְּׁלוֹשִׁ֗ים הִנּ֤וֹ נִכְבָּד֙ ה֔וּא וְאֶל־הַשְּׁלוֹשָׁ֖ה לֹא־בָ֑א וַיְשִׂימֵ֥הוּ דָוִ֖יד עַל־מִשְׁמַעְתּֽוֹ׃ (ס)
Siehe, er war ehrenhafter als die dreißig, aber er erreichte nicht die ersten drei; und David setzte ihn über seine Wache.
Rashi
Of the thirty, he was the most honored, but to the three he did not come To the heroic feats of the three he did not reach; i.e., to those three mentioned above (verse 20). Therefore, לא is written above with an “aleph.”
Rashi
over his guard עַל מִשְׁמַעְתּוֹ, an expression of appointment, and likewise, (Isa. 11:14): “... and the children of Ammon will be their appointees מִשְׁמַעְתָּם.”