Numbers 12:4

Suddenly. He revealed Himself to them suddenly, when they were ritually unclean because of marital relations, and they cried, ”Water! Water!” To inform them that Moshe acted properly in separating from his wife, since the Divine Presence revealed Itself to him constantly, with no designated time for the Word.

Come out, all three of you. This teaches that all three were summoned with a single word, which is impossible for the mouth to say and the ear to hear.

And the Lord said to Mosheh, to Aharon, and to Miriam, Come forth, you three, to the tabernacle. And those three went forth.

AND THE ETERNAL SPOKE ‘PITH’OM’ (SUDDENLY) UNTO MOSES, AND UNTO AARON, AND UNTO MIRIAM. Now Moses was not with them, but [they are mentioned together because] the Divine communication came to the three of them simultaneously. The sense of the word “suddenly” is that they did not direct their minds towards or intend to receive a Divine communication at that time, it being in honor of Moses that it came to them without any preparation for it; for the word pith’om (suddenly) in the opinion of the commentators [as explained in Ibn Ezra] applies to something which one did not think of, from the root pethi (simple-minded). Therefore Scripture uses the term pith’om [only] on account of Aaron and Miriam, for Moses our teacher was fit for a Divine communication at any time, and his mind was prepared to cleave to the Glorious Name at every moment, as our Rabbis have explained in connection with the reason why he [Moses] separated himself from his wife. Onkelos, however, rendered [the word pith’om] as bithkeiph (in a hurry), the sense being that whilst Miriam and Aaron were still speaking about Moses, and the words were still in their mouths, they were told: Come out ye three unto the Tent of Meeting, and He did not delay [the rebuke] to them at all. The word pith’om is thus a term indicating hurry. Similarly, And I beheld his habitation cursed ‘pith’om’ (suddenly); whose breaking cometh ‘pith’om ‘l’pheta’ (suddenly at an instant); ‘b’pheta’ pith’om’ (very suddenly), the double expressions being for emphasis, just like: ‘kim’at kot’ (a very little while); ‘harbei m’od’ (exceedingly great). So also: ‘bi’m'od m’od’ (very much),, 17:2. and similar cases. And pethaim (simple-minded ones) are those who are very impetuous, who do not have any deep grasp of a matter and do not reflect on it at all, the usage being similar to the expression and the counsel of the wily is hurried. So also: ‘petha’ (on a sudden) shall he be broken; and if ‘b’phetha’ (suddenly) without enmity, [which Onkelos renders] bithkeiph, which is like pethi (simple), of the expression pith’om (hurriedly).
The reason [why He said at first] Come out ye three and [then in the following verse it says] and He called Aaron and Miriam [excluding Moses] is that G-d wanted him to be present [in the Tent of Meeting] and to see how He is zealous for Moses’ honor; and so that he would be available [to forgive them], for G-d would not forgive them unless he did, after they would beg him and he agrees to [forgive] them. And He called Aaron and Miriam, in order to tell Moses’ praise when he was not present.

פתאום, at the very moment when they were speaking about Moses. This was not a customary hour for G’d to communicate with them, but seeing G’d was upset about them on behalf of Moses, He wanted to pay him the honour of reacting immediately.

צאו שלשתכם. G’d wanted that Moses should become aware of His being concerned immediately when anyone impugned his honour.

פתאום, “suddenly;” G–d reacted so immediately in order that Miriam and Aaron could not say that G–d’s reaction was due to Moses having complained about being slandered.