Here in Boston (at the Boylston store) as far as I'm aware they never sold out--at least not early in the day. We got to the Apple Store at 815am and there was a line but nothing too long. By 9am the line grew a lot but the only thing that delayed us was that a lot of people had made reservations and were allowed to enter the store first. Nonetheless we got into the store by 10am and they had plenty of stock even though we didn't end up purchasing one. This was on a Saturday.
I think less people are going to be going on a Friday evening than would on a Saturday morning. Also, there are no reservations for the iPad 3G so the line should move much quicker. On top of that, the cheaper iPads will likely sell more so I don't see demand for the 3G model being as high as that for the WiFi-only model. These three things combined make me think that you should have no problem getting an iPad 3G wherever you go.
Having said that, I'd say go to the larger store and get there by around 3:30pm and I think you'll be fine. That's just me 2c though.