A bank submits multiple unauthorized transactions to their third party for investigation. They receive results back on different dates indicating specific disputed charge are closed. For example, customer disputes 3 charges ($500, $300 & $200) on 6/1/23 and bank gives notice of provisional credit of $1,000 on 6/5/23. On 6/15 they get a response that the $500 and $300 charges that were disputed is completed and closed. On 7/7/23 they get notice that the $200 charge is now completed and closed. Is the bank required to report the results of the $500 & $300 within 3 business days of 6/15 or can they wait until the final disputed charge of $200 was completed on 7/7/23 to provide final notice?

I have not seen anything on this but would not complicate the issue by subdividing the claim. That is, it's multiple transactions but you're handling it as one claim. You may get different results on each transaction, but there is still one claim. When the claim is finalized, notice is required.

I will add that one claim is supported by the fact that you do not apply liability rules against each transaction.

Although the card processors certainly charge us the fee per item to process the disputes on them!

Another good example of how Regulation E and card association rules are separate and different.