The only way you're going to be able to do this is if you add the comID column to TableA, insert into that, and use the INSERTED.comID to insert into TableB. The output clause will only see the columns actually being used in the table being inserted into ( @TableA ), and you aren't allowed to join back to TableC.
Wayne
Microsoft Certified Master: SQL Server 2008
Author - SQL Server T-SQL Recipes