Thanks for posting here in the Community space. Let us give you insights into importing your client's payment in QuickBooks Online using TPI.
When importing transactions to QBO, it's essential to note the limitations of the number of transactions that can be imported. Specifically, QBO has a limit of 1000 rows of transactions. Since you're using a third-party integration (TPI) to import transactions, there is a limit of 4MB with 5000 rows of transactions. You may have reached the limit of importing the transactions to QBO since you mentioned that it's a three years' worth of data. Therefore, I suggest checking the number of rows to see if you have reached the limitation. If this is the case, you can split the importing of transactions into smaller batches to ensure that it will import everything you need.
We suggest you share the error message if you have not reached the limitation so we can assist you in resolving the issue.
On the other hand, you may also use these articles as a guide when running a report that may help you with your business inside QBO:
Let us know if you have additional queries about importing your client's payment in QBO. The Community space is here 24/7. Keep safe!
