John Gill Commentary Daniel 1:18

"And at the end of the days which the king had appointed for bringing them in, the prince of the eunuchs brought them in before Nebuchadnezzar." — Daniel 1:18 (ASV)

Now at the end of the days that the king had said he should bring them in That is, at the end of three years; which was the time appointed for their education, and when they were to be brought before the king for his examination and approbation: then the prince of the eunuchs brought them in before Nebuchadnezzar ; even all the young men that were taken from among the children of Israel and Judah, as well as the four children before and after mentioned, appears by what follows. This was done by Asphenaz, and not Melzar.