Creating virtual keys to join by

I am always needing to do something like this, but can't seem to find the right approach:

Summary: Join 1 table to itself to summarize data based on a substring of one of the fields. I have read-only access to the data.

select unique substr(field1, 1,3) site , count(field1) mycount from db.table1 t1 join db.table1 t2 on site = substr(field1, 1,3) group by site;

The problem is that "SITE" is an invalid identifier in the join and group clause. Using SQL*PLUS with Oracle 9i

Regards,
Nick
Example Data

Field1
ABC223432432
ABC234299392
ABC899898988
BBC230293090
BBC2320020202
BBC293920020
BBC44343433 Results Expected
ABC 3
BBC 4 Received on Sat Jul 23 2005 - 10:51:46 CDT