Currently, there isn't a way to upload the customer's custom fields in QuickBooks Online. As a workaround, I'd suggest utilizing the CustomerType feature. I'm happy to provide detailed steps and screenshots to accomplish this.
Let's create a type first by going to Sales and choose the Customer tab. Then hit Customer Types and enter a name.
After that, you can now assign the type to your three members by following the steps below.
Select Sales, then choose Customers.
Check off the three customers and hit the Batch actions drop-down.