1. generally, you will find that 500-1000 rows/transaction will give the optimal performance. Beyond that, you won't see much performance improvement.

2. doing all 3 million rows in the transaction adversely impacts server resources and performance.

3. If the upload table is going to be shared, you might also consider doing individual uploads to separate tables (one per user) and then executing an Insert statement from the upload table to the shared table -- cached upload.