Just thought I'd add to this thread for those who are running OTA 2.08.651.3 (I just got my EVO 3D a few days ago and that's what was on it (I think the guy behind the counter updated it without asking me)) After rooting the phone and reading through a few threads I was unable for the last few days to get wifi tethering to work. I did finally just a few hours ago get it working. Here's how:
Get latest version of Wifi Tether (Currently 3.1 beta 6) and install.
(YOU MAY BE ABLE TO IGNORE THE BARNACLE STEPS, TRY JUST INSTALLING AND RUNNING THE LATEST VERSION OF WIFI TETHER WITH PROPER SETTINGS 1ST!)
Get Barnacle Wifi and install.
Open Wifi Tether press Menu > Settings > Check Routing Fix and MSS Clamping (my 3G wouldn't work very well without it)
Also under Change Device-Profile select HTC Evo 3D. Thanx @CarrieK!
Start Wifi Tether and Touch screen to Start Tethering and accept Superuser. Then again to Stop Tethering and accept Superuser.
Now Start Barnacle Wifi and Touch Start. If you get an error about wap_***** just accept ok so that it will run anyway. (If the Start button doesn't change to Stop you did it wrong, just run it again) Accept any Superuser requests.
Now Touch the Stop button in Barnacle Wifi. Accept any Superuser requests.
Press and hold the Home button or drag the taskbar down so that you can see your recently used applications (leaving Barnacle running) and select Wifi Tether again.
Go to Menu > Settings and Enable wifi encryption if you wish and set your passphrase etc. You may also Enable Access-Control.
Make a 4G connection (set phone to 4G). Check to see if it is working.
Stop Wifi Tethering and set phone to 3G and restart Wifi Tethering. Check to see if it's working.
Stop Wifi Tethering. Stop Barnacle. Uninstall barnacle. Use Wifi Tethering normally.
This is what worked for me. Hope it helps someone else out.