Garages and Pole Barns Page May Also Provide Some Information for you.
If the three 2 x 6's referred to are the Newer Style of Treated Material on the bottom with regular SPF Spruce Pine Fir above ground this material of glued boards May be stronger, straighter, and better than solid posts.
There are different anchoring Methods if using the concrete.
Some are worse than posts sunk, some may be close to the same for initial strength. The advantages of the anchors are it may be easier to keep straight and the prevention of rotting from moisture content flexing long term.
Yes - Your ground Level Moisture Content may also be a factor.
Talking With Your Local Building Inspector may be Your Best Source Of Information for your area.
About Home Plans For Free and how find the right home plan.
Garages and Pole Barns