Let me help you record your transaction, @csm8396.

You can send an invoice to your customers for products and services that you sell to get paid in the future.

However, if you have customers that have an existing balance or a past due amount, the feature to automatically include the amount of the previous invoices balances to the current ones are unavailable.

I suggest sending your clients a reminder message about an overdue invoice. Here's how to send it manually:

1. Go to the Sales menu and click on Invoices.
2. Locate the invoice you'd like to send a reminder for.
3. From the Receive Payment drop-down, select Send reminder.
4. Customize your message and select Send. 

Check this article for more information: Automate invoice reminders in QuickBooks Online. When you receive the payment, you can record invoice payments manually.

For a single payment that you've received from 3 different clients, you can add them as Bank deposit. 

When you create a bank deposit, add those 3 clients in the Add funds to this deposit part and the amounts they’ve paid. You can check this article for more information: How to Record Bank Deposits in QuickBooks Online. 

Keep in touch if you still have additional questions with your invoices. I'm always here to assist. Have a wonderful day!