As Ken notes above, you need the agent installed on each machine.
From there, when creating a job you will need to create a logon account for each connection since they are in three seperate domains. When making backup selections, you will be prompted for a logon account which has rights to that machine. Once you create the proper logon account, the job will use it every time it connects to that machine.
Your backup jobs can contain multiple logon accounts for different resources.