We are a group of 3 different businesses with corporate free accounts, 

we registered each account with each company email: 

xxx@domain1.com, xxx@domain2.com, xxx@domain3.com

Is there any easy way to manage these 3 different emails from one place?

Maybe if we add the same user to all 3, that user will be able to manage them all from one account?

Is it required since they are 3 different domains to join some paid membership?

Thanks