I have 3 different business (DBA's) that operate under one corporation. All 3 of my businesses are run by me and one assistant. Can I use 3 customized invoices?

I'm glad to see you dropping by here in the Community, @robin37.

In QuickBooks Online, you can create customized sales forms up to the limit of 25. You can make 3 separate templates and use them when entering customer invoices.

Here are the steps:

1. Click the Gear icon in the upper-right corner and select Custom Form Styles.
2. Select the New style drop-down.
3. Choose Invoice. 
4. Customize your invoice template.
5. Tick Done.

Additionally, If you're using three different EIN, it's recommended to use one subscription per business. That way, financial data will be accurate, and filing tax returns will also be correct. You can seek expert advice from an accountant to guide you with how to set up these businesses under one corporation.

Furthermore, I suggest checking out the Customize invoices, estimates, and sales receipts in QuickBooks Online article. It contains additional insights about customizing invoices with some visual screenshots.

Looking forward to your reply. Thanks in advance. Have a great rest of the day.