The \1 etc. replace expressions, and in order to specify that something is an expression, you enclose it in (). So you need
([A-Z])([0-9])([A-Z])([0-9])([A-Z])([0-9])
Then
\1\2\3 \4\5\6
should work. I think there is a limit on the number of expressions you can have (9, perhaps)