I do my body fat testing with calipers and I believe that the more sites you use, the more accurate the results. The 3-site test is easy to do , but you might need someone to get the tricep measurement for you.
There is a great site that gives instructions on taking these measurements and it also will automatically calculate your BF % using the measurements that you plug into the calculator.
www.enforcergraphics.f2s.com/bodyfat.htm
The reason I think that the 3-site is better than just the suprailiac is that people carry their weight in different areas. My problem areas are the hips and thighs. If I did not include the HIGH thigh measurement, it would show that I was leaner than I really am. Conversly, my friend has no hips and has very lean thighs but she carries her weight in her waist and upper body. If she only used the suprailiac, it would not average in her low-fat thighs.
Of course, this is just my $.02.