Assuming they're referring to size, 3u is 3 rack units high. 1 Rack unit is 1.75". This is of course referring to the standard 19" rack that is found in server and pro audio environments. Here are some pics to help.
An audio rack:
A 1u piece:
A 2u Piece:
A 3u Piece:
A 4u Piece:
That help explain things better?
