Sure, that is possible. The problem is, when the other two Zune "cards" or "tags" log in with their username and password, they cannot download anything. To download, they need to log in with the "master" login info. Once downloaded to their computers, they can log back in with their login info, and play those songs.
The downside of that is that the "master" account can only be logged in one time simultaneously, so if you and two kids want to download, one has to get in, download, and log out to free up the slot for someone else.
What invariably happened to us was the kids would forget to logout and that blocks everyone else, not to mention their teen metal and pop was showing up on my Zune card.
Ultimately, I wound up getting two Zune passes.