A few more details would be nice in order to provide a complete answer. You say you have three domains and that trusts have been established among the domains. For the sake of argument, let's call the domains A, B and C. Let's also say that SQL Server is running on a server in domain A. Finally, let's say that you have two users you want to add:
With Windows accounts, you don't create the account. The account already exists in the OS. Instead, you GRANT the windows account access to the appropriate database. See SQL BOL sp_grantlogin.
Note that B\UserName and C\UserName are not the same entities; they may share a common UserName, but the are distinct and separate. Pass-through authentication does not apply here since these are domain accounts.
If this does not help you to solve your issue, may I suggest that you post the actual details of your situation so we can understand the issue better?