I have two files that I use to play TS3 without Origin. However, I purchased and installed them prior to Origin (but I am able to play through Origin if I want). I don't use the expansion directory; my files are in: \\\Program Files (x86)\Electronic Arts\The Sims 3\Game\Bin\[file]. I use TS3.exe to by-passes the launcher, or I use Sims3Launcher.exe to see the launcher first. If you purchased/installed through Origin, I have no idea if you can by-pass it.
MrsFlynn's instructions are perfect for going offline, but there's a way to launch Origin in off-line mode to start if that's your preference. You can temporarily disable your internet (easy on a laptop, not so sure about a desktop) and launch Origin. It will notify you that you are offline and ask for you to log in (even though you aren't online). You can then re-enable the internet and Origin will remain offline. Another alternative is to have Origin logged in on another computer and then launch it; you'll be notified that you're logged in elsewhere and asked if you want to continue in off-line mode (my daughter and I experience this all the time).
