Three days (hēmerai treis). A parenthetic nominative (Robertson, Grammar, p. 460).
What to eat (ti phagōsin). Indirect question with the deliberative subjunctive retained. In the feeding of the five thousand Jesus took compassion on the people and healed their sick (Mat 14:14). Here the hunger of the multitude moves him to compassion (splagchnizomai, in both instances). So he is unwilling (ou thelō) to send them away hungry.