Troubleshooting: From User Properties > Attribute, we can see all 3 email addresses are there.
However, Aliases under Office 365>admin center>Active users shows only two email addresses.
Go back to User Properties > Attribute, we find smtp: and b.com email address has a space on username, for example smtp: username@b.com. Changing it to smtp:username@b.com and re-syncing fixes the problem.
