You can add as many OneDrive accounts as you can through the OneDrive app, but if you noticed you'll need to be signed in in all OneDrive accounts which is managed directly from the OneDrive app.
Having 3 different accounts connected simultaneously via the Acrobat Reader mobile app is not a possible workflow, unless you make sure that you're signed in in all of those accounts from the OneDrive app itself.
To access your OneDrive accounts from the mobile app you may also need to setup your OneDrive app as a local file location in the "Browse for more files" menu options.
When you tap on this option you will see all the active ("currently connected") cloud service accounts in the next screen "Open From".
Tapping on one of those OneDrive connections will bring you to that OneDrive cloudspace
folders to navigate through your files.
Last you won't be able to open PDFs directly in the Acrobat Reader mobile app if you tap directly on those files. You must download first to your mobile device and save it to a known location in order to be able to open them with the Acrobat Reader mobile app.