Yeah, I'm with Revvy. I take one right before pitching the yeast, then another right before bottling/kegging.
I think that there isn't a reason to take three in a row to double check it's not changing, IF you leave it in the fermenter long enough, AND if it's in the expected FG range.
For example, I brew a 1.070 IPA. Four weeks later, it's at 1.012. Based on the yeast, I used, I can expect a 80% attenuation. Well, that's exactly what I have there- and the beer is clear and finished. If I was in doubt (maybe if it was only at 1.020, or the beer was still having krausen), I'd wait and check it again in a few days. But if you are pretty certain it's finished, an SG check is the perfect confirmation.