וַיֶּאֱסֹ֥ף אֹתָ֛ם אֶל־מִשְׁמָ֖ר שְׁלֹ֥שֶׁת יָמִֽים׃
Et il les garda en prison durant trois jours.
Rashi
and the evening depart When will it become light and when will the time of arising come, and when will the evening depart? [The word מדד ] is an expression of (Gen. 31:40) “and sleep was banished (ותדד).” The “mem” is a defective radical, as in מאמר, statement; משמר, watch.
Chizkuni
לשלשת ימים, according to the plain meaning of these words the meaning is: “in anticipation of the third day.” According to the plain meaning, Moses did not add an additional day of preparation (sanctification) as suggested by some scholars. Other examples of a similar formulation in the Bible as here are Genesis 42,17: ויאסוף אותם אל משמר שלשת ימים, “Joseph put them in jail for three days.” Or, in the verse following: ויאמר אליהם יוסף ביום השלישי, “Joseph said to them on the third day;” or Genesis 40,19: בעוד שלשת ימים, “within another three days;” and in the verse following: ויהי ביום השלישי, “it was on the third day;” or Exodus 19,16: ויהי ביום השלישי, “it was on the third day;” there are numerous other examples.
Rav Hirsch
(21-34) Alle diese Verhandlungen waren wohl während der drei Tage ihrer Haft zwischen ihnen und Josef vorgegangen.