"I have completed three of the reports, as attached. The balance of the reports will be sent soon"
We use written words for small numbers, and adding a leading zero is rather confusing.
Jeffs 'the three' doesn't quite make sense though. It isn't clear which three you are referring to. You could write 'the first three' or 'these three', or as I suggested 'three of the reports'. 'The balance reports' also isn't very good English. You need either 'the balance of the reports', or you could use 'the remaining reports'.
'Will follow' is possibly a little curt - not really impolite though.