What are the implications of the word "Pis'om"?
Rashi and Ramban #1: It implies the call came suddenly 1 and caught them by surprise.
Ramban: Because the root of "Pis'om" is 'Pesi', with reference to something that did not enter one's mind. Refer also to 12:4:2:2, and the Torah uses the word in connection with Aharon and Miriam, because Moshe was prepared for Nevu'ah at all times.
Why did Hashem do that?
Rashi: He did it in answer to their criticism of Moshe
What are the implications of the word "Sheloshtechem"?
Rashi: It implies that Hashem called Moshe, Aharon and Miriam independently all at the same moment. 1
Rashi: Something that no human-being can possibly emulate.
Why did Hashem call Moshe?
Rashi (K'sav Yad Teiman): Although Hashem did not speak with Mohe, He wanted him to be there in order to Daven for a Refu'ah Sheleimah for Miriam.
Ramban: He wanted Moshe to be there when he scolded Aharon and Miriam, to see how He (Hashem) was zealous on his behalf, and for him to be available to pardon them, since He would not do so without his consent, when they would plead with Him and He would be appeased.
Seforno: Because He wanted him to know about His goodness, how He cared about his Kavod.
Rashbam: Because He was scolding Aharon and Miriam on account of Moshe, and to give Moshe Kavod.