We have two paid accounts with you,we would like to add a 3-rd. Is it possible to do it for free or link it to one of the first two accounts, so that it would be free?

You'll need to have a separate paid subscription to add a new account, @pavlos-gabaeridi.

This is because QuickBooks Online (QBO) is a per company file subscription. Though, you can access them all with the same login info. Doing so lets you quickly switch between companies and manage everything more efficiently.

For more info about, please review this resource: Create or add another QuickBooks company.

Also, you're able to personalize QBO to work the way you want it to. To accomplish this, you can simply turn features on and off on the Account and Settings page. Please check out this article for the detailed steps: Edit company settings.

I'm just a comment away if you have more questions. I've got your back. Thanks for dropping by.

@pavlos-gabaeridi 

One QBO account is for one company file.

https://quickbooks.grsm.io/EU

https://quickbooks.grsm.io/South-America

Consider switching to QB Desktop to manage multiple company files with a single one time license. Deploy a private cloud if required.