Hello there, Vero. I'm here to make sure you'll be able to add a 3-letter acronym to your Purchase Order in QuickBooks Online (QBO). 

Let's enable the Custom Transaction Numbers feature from the Account and Settings. This way, you can customize the Purchase Order (PO) numbers to include a 3-letter acronym to identify each project. 

Here's how:

1. In your QBO account, proceed to the Gear icon. 
2. Select Accounts and Settings. 
3. Choose the Expenses tab.
4. Click the Purchase Orders section.
5. Turn on the Custom transaction numbers option.
6. Once done, select Save and Done. 

Then, let's open the purchase order and modify the number to 1005CDO or CDO1005. 

I've added this article to learn moew about how to add accepted purchase orders to transactions for your vendor:  Add purchase orders to expenses, bills, or checks in QBO.

Keep your posts coming if you need more assistance in managing your purchase order in QBO. I'm always here to help. Have a great day.