My Lightroom 3 just stopped tethering recently, but I can tether with a trial version of Lightroom 5 with no issues at all. What seems to be the tethering issue with Lightroom 3?
Please refer the list of support cameras for tethering in Lightroom 3. A list of cameras that can be tethered to Lightroom.
Also you can reset the Lightroom preferences.
To reset your preferences, do the following:
Navigate to the preferences file:
Windows 7, 8: C:\Users\[user name]\AppData\Roaming\Adobe\Lightroom\Preferences\Lightroom 3 Preferences.agprefs
Mac OS: /Users/[user name]/Library/Preferences/com.adobe.Lightroom3.plist
rename the Lightroom 3 Preferences.agprefs/com.adobe.Lightroom3.plist to Lightroom 3 Preferences.agprefs.old/com.adobe.Lightroom3.plist.old
Let us know if that helps.