Also, unless you need to free up the primary fermenter, I'd just leave the beer alone for the entire 3 weeks. Under ordinary conditions, there's little reason to move a beer to secondary.
For standard mid-gravity beers, I don't bother much with FG readings if the fermentation was "normal." I suppose I usually measure it once to be sure it's where I expect it, but unless it's suspicously high, I don't take the second one to check it. If you've got a big beer that you expect to stop at a gravity over 1.015 or so, the check is more important. But after 3 weeks, if the gravity is down below that, it's very unlikely that well-managed fermentation is incomplete.