It's pretty simple explained: <Track#:3> results in a track number with 3 digits: 312
also (probably a bug): in file organizer the track# is not handlet as number.
Also in file organizer, filter "Album track count > 99" does not work.
So it seems no way to do a 3 digit automatic formatting when i got more than 99 tracks for an album except the "renumber tracks.." tool, which just renumber the tracks through and does not reformat the current number.