where was Daniel ?

He was not with the three others. The end of chapter 2 reads

Then the king placed Daniel in a high position and lavished many gifts on him. He made him ruler over the entire province of Babylon and placed him in charge of all its wise men. Moreover, at Daniel's request the king appointed Shadrach, Meshach, and Abednego administrators over the province of Babylon, while Daniel himself remained at the royal court.

I am not sure where the royal court was, but it was evidently not in Babylon proper.

Here is a general map of the entire Fertile Crescent:
http://www.bible-history.com/maps/maps/map_ancient_fertile_crescent.html

The city and province of Babylon are to the right, where the two main rivers are fairly close together. Ninevah, Damascus, and a number of other ancient and major cities were quite spread out along that crescent. Here is an idea:

http://www.piney.com/BabMap.gif

You can see that Babylon is separate from Assyria. My guess is that King Nebuchadnezzar may have set up his court in one of the main cities of the northern area in Assyria. Someone else may know this part of history better than I do, though.