The difference between these two types is due to the material used to manufacture them.
Type 1 is a carbon or alloy-based steel that will rust and corrode over time if exposed to natural elements. The Type 3 material is a weathering steel that has natural atmospheric corrosion resistance properties and will rust over time as a way of protecting the steel from corroding. A449 type 3 fasteners are not readily available in the marketplace due to a lack of raw material availability.