Certainly an Internet connection is required to install the Steam versions of TS3 and its EP/SPs and run them the first time for authentication purposes, but not from that point forward. Steam delivers its own build of Patch 1.67 and therefore a Launcher bypass using a shortcut/alias to TS3w.exe is still sufficient to launch the game after the first run. You will however also need some sort of Internet connection to obtain store content all over again as the game won't let you use store content downloaded under an Origin/disc account on the Steam platform, but that's just another form of one-time pain rather than something ongoing.
This may change sometime next year when Steam begins expanding its EA titles and there may be more of a Steam-Origin connection than the none, not possible, don't need it anyway one we have now. It remains to be seen what will be required of us TS3 players on Steam once that happens.
Of course, not that this is much help for today, it's also possible that Origin may someday fix its Play Offline feature so that it works closer to what was probably intended.
