Its all to do with the target line in the client icon
Right mouse click on the icon and make sure it has this
Ealand client
"C:\Program Files\Maxis\The Sims Online\TSOClient\TSOClient.exe" -w -r800x600 "-CSCity:EA Land"
TC3 client
"C:\Program Files\Maxis\The Sims Online\TSOClient\TSOClient.exe" -w -r800x600 "-CSCity:Test Center 3"
this is my target line, yours may be slightly different in that it may not say Maxis
the important bit is the bit that comes after .exe"
If you play both EALand and TC3 you will need 2 seperate client icons