Rashi on Genesis
שלשת ימים ARE THREE DAYS — They are a symbol to you of three days. There are many Midrashic explanations of these words (cf. Chullin 92a).
Radak on Genesis
שלשת ימים, when reflecting on the meaning of the dream Joseph realised that whatever it presaged would occur very soon. He surmised this from the speed with which the blossom on the vine in the dream had turned into fully ripened grapes and wine. He therefore interpreted the number 3 which occurred in the dream as not referring to years or months but to days.
Siftei Chakhamim
They are a symbol to you of three days. I.e., they are not three days themselves, as the verse seems to imply.