One scholar, Haley (Alleged Discrepancies, p. 248), makes this observation with regard to this:
“Moses, under the impulse of the Holy Spirit, was writing history 'objectively.' Hence he speaks as freely of himself as he would of any other person. It is also to be observed that he records his own faults and sins with the same fidelity and impartiality.”So far as Numbers 12:3 is concerned, it should be observed that Moses' failure to speak in his own defense, even when put under great pressure by Aaron and Miriam to lose his temper, calls for special explanation. That explanation is found in his complete deliverance from pride and his thoroughgoing commitment of himself to the Lord God as his vindicator and protector. Any other leader in his position would surely have faced them with a withering reply, but Moses turned the matter completely over to God. We really need the information contained in v.3 in order to make sense of his amazing meekness in this situation.
–Dictionary of Bible Difficulties