The only way to have more than 3 on server 2003, is to change your terminal services mode from remote administration, to application mode, for this you also have to install terminal services licencing. Once this is installed you need a client access licence for each client that connects.
Depending on the licencing mode you choose, these licences are either allocated per machine, or per user. So if you have users using multiple machines, your better off going for the latter.
Finally, you do get a 90 day temporary licence issued to users if you have no licences, which will allow access for 90 days, after that they cannot connect without a licence being availible.
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