Somebody told you it would require 3 feet of concrete?
Typically a pole barn has posts dug into the ground, and then those are filled in with concrete. A typical concrete slab for a pole barn would be 4". There is no trench around the building.
Typically the post are NOT filled with concrete, but native soil/gravel. No reputable polebarn builder around here will fill along side the wood post with concrete. There will be a concrete footer or cookie. Most will even isolate the slab from the post with some plastic or 1/4" xps foam.
Nearly all builders have a precast concrete product that is similar to the permacolumn. Some such as mortons, Does have the post bottom set in wet concrete but concrete is only to be placed to the bottom of the actual post. Or in other words, the post itself is not installed in the concrete just the steel stirup. This stirup sets the height of the post, and provides uplift protection attached to the poured footer. Permacolumns typically are not set in concrete, but they do sell the stirups if you so desire.
Uplift protection on wood post and the standard permacolumn is handled by uplift cleats attached to the post near the bottom. They are really only needed for the first year or so. After that soils settle and hold the post firmly. Most buildings sucked out of the ground by high winds happens relatively soon after construction.
Both post-frame construction and steel/red iron can be built upon full foundations, with or without stem walls, too. It's all about your end goals, preferences, and what the AHJ requires.
Just to make a significant note if using wet set brackets on post frame foundations. The typical full post buildings take significant bending moment in a pole barn reducing the need for "as much" shear diaphragm's. The wet set brackets do not transfer this bending moment.
Quoted from the Sturdywall plus design manual.
"The standard Sturdi-wall brackets are designed to transfer uplift, shear, and downward forces from the wood post into the concrete foundation and can be installed wet set or post drilled anchors.
The standard Studi-wall brackets do not have an ability to transfer any measurable moments from the wood columns to the concrete foundation."
If such wet set brackets are used, the diaphragms in the building must typically be made stronger, with diagonal bracing/strapping or OSB/Ply shear panels in addition to the metal sheathing. This keeps bending moments from happening and transfers it all as shear which the brackets do very well at.
