Chabad.org — English translation + Rashi
Torah Temimah on this passuk (citation pointer only). Core text is the Chazal reading that “פתאום” can denote ones (unavoidable suddenness).
English: “And Hashem said suddenly.” From here: “pitom” denotes ones (sudden compulsion).
Lexical rule — pitom marks an event that strikes without prior preparation.
The sudden call of the three to the Ohel is the model for the word’s force in other contexts.