After signing-in to Azure DevOps Services, on the left pane I can select from three different accounts:
All these accounts are pointing to my name and my e-mail address.
What's there reason I have three different accounts? How can I delete the redundant ones? Is there any documentation available, explaining this odd situation?
The aex signin page (https://aex.dev.azure.com/) lists the Organizations your account has some access to.
You should be able to see the list of organizations after logging into the MyAccount page with your Microsoft account, and you can leave the Organizations you don't need:
https://myaccount.microsoft.com/organizations
These are separate from Azure DevOps Organizations, despite having the same name. It's sort of organized like: Microsoft Account > Organization (Azure tenant) > Azure DevOps Organization > Project
You can skip straight to your DevOps project by using a link like:
https://dev.azure.com/MyAzureDevOpsOrganization/MyAzureDevOpsProject
