Are you referring to having it in one subscription? If so, the feature to merge all your QuickBooks Online (QBO) account in one subscription is still unavailable. One QBO is equivalent to one business.
What we currently have is the ability to use one email address for all your account and easily switch between companies any time by going to your Settings.
First, choose an email that you want to use for all the account.
Then, log in to the other accounts that is notusing this email and add or invite it to your account as the Company Admin.
From here, change the email and user ID for the other two QBO accounts.