All methods of skinfold tests are somewhat inaccurate -- they're an estimation.

It makes sense that a seven site measure would be slightly more accurate than a three site measure, but I think a three site measure is fine for your purposes.
http://www2.gsu.edu/~wwwfit/bodycomp.html
Skinfold tests are good for tracking trends if you have a decent caliper. I've had the kind I squeeze myself and always felt like I never knew if I was squeezing harder in one place or another. It's difficult to know whether the pressure you're applying is constant or not.
There are more expensive calipers (a lot more expensive) that apply a constant standard pressure (Lange, Skyndex, Harpendon, etc.)
http://www.drwoolard.com/fitness_tes...d_calipers.htm
If you check out Exrx, they have a good explanation for how to take measurements, sites and a calculator:
http://www.exrx.net/Testing/SkinfoldProcedures.html
Here's another site that shows photos of where you should pinch and explanations of how you should pinch:
http://www.topendsports.com/testing/skinfold-sites.htm
For accuracy purposes, it's also important to figure out a way to make sure you pinch the same places on your body each time. It's difficult to take your own tricep or subscapular measurement.