Can I create one invoice and split the payment due dates into 3 payments? It's a long term project and we've agreed phased payments each month, is this possible?

Welcome to the Communtipace, @iamcamilla-hotma. 

Right now, creating one invoice can only have one due date. As a workaround, you can record an estimate first. After that, manually add the amounts to an invoice one at a time so the due date will be achieved.

To create an estimate, here's how:

1. In the left menu, click the + New button.
2. Select Estimate.
3. Choose the customer's name, then fill in the necessary information.
4. Add the total amount under the Amount column.
5. Click on Save to record the transaction.

Once it's completed, convert it to an invoice:

1.  In the left menu, click the + New button.
2. Select Invoice.
3. Choose the customer's name under the Customer section.
4. Look for the estimate at the right side of the page.
5. Tap on Add, enter the amount on how much do you want to invoice on the pop-up window.
6. Click Copy to invoice.
7. Save the transaction.
8. Repeat the steps for the rest of the amounts.

For more reference about converting an estimate to an invoice, read this article: Convert an estimate into an invoice in QuickBooks Online.

For future reference, here's an article you can visit about the different reports you can run to give you a snapshot of your business: Run reports in QuickBooks Online.

Know that I'm always around the Community to help you. Click the Reply button if you have other questions about in QuickBooks. Have a great rest of the day!