The steps shared in this thread is for QuickBooks Deskrop, Sstoro.

Also, you only need to delete one payment if it was applied to the 3 transactions. Then, the invoices will automatically be opened again.

Let me share the steps on you how can do this in the online version. Here's how:

1. Click the 1 payment made link in the upper-right hand corner of the affected invoice. This is just below the PAID status. 
2. Click the date of the payment.
3. Once the payment is opened, click More at the bottom and select Delete.
4. Click Yes. 

Please take note that deleting a transaction in QuickBooks Online can not be undone. 

You'll also want to run the Invoices and Received Payments report. It lists all payments you’ve received and the invoices that go with them. Just go to the Reports menu and type in Invoices and Received Payments in the search bar. 

Let me know if you need more help while working in QuickBooks Online. I'd be glad to get back here and offer my assistance again.