I understand the challenges you're facing with the upcoming end of life for your Desktop Pro 2016 and the need to manage the books for three separate entities, baker_eliz.  

The option to add multiple companies under one subscription isn't available in QuickBooks Online (QBO). Each company you create requires an additional subscription, but you can access them with the same sign-in info. This lets you quickly switch between companies so you can manage everything more efficiently. 

When adding a new company, you can visit our QuickBooks pricing page and select the subscription option you want. 

Now that you have multiple companies, you’ll be asked which company you want to open whenever you Sign in to QuickBooks Online. To switch between companies at any time, select the Gear icon and select Switch company.

To learn more about managing multiple companies, check out the following link. It has a bunch of useful information for you and your business: Managing your companies. 

Please let me know if there is anything else I can do to help you with managing your subscription or account. I'm available to help you at all times. Take good care.