Charles Spurgeon Commentary Daniel 1:5

"And the king appointed for them a daily portion of the king`s dainties, and of the wine which he drank, and that they should be nourished three years; that at the end thereof they should stand before the king." — Daniel 1:5 (ASV)

And the king appointed them a daily provision of the king's meat, and of the wine which he drank:

Treating them exceedingly well, thinking, perhaps, that the very food they ate might help to tone their minds for the work to which he had called them. He wished to make them into true Chaldeans, so he ordained that they must eat of the meat he ate, and drink of the wine he drank.

So nourishing them three years,

Putting them to college, as it were, for three years.