If it is structural concrete, as opposed to slabs intended to remain on grade, and is cast against soil or permanent fill, the requirement is 3 inches, because: 1) soil formed surfaces are ALWAYS irregular, 2) bar supports may embed into the soil reducing minimum cover (use sand chairs or precast blocks), 3) soil carries moisture to the concrete. The cover is intended to reduce corrosion rates, therefore maintaining adequate cover is imperative where it is cast against and remaining in contact with soil.
Alternative means of protecting the concrete are permitted by 318. If the barrier (even a plastic sheet barrier) is adequate to stop migration of moisture up to the concrete surface, and will not act as a collector of moisture holding it against the concrete surface, it may reduce the required cover to 2 inches or less.
Since you are on piles, it is NOT a slab on grade, since it will need to span between the piles if there is subsidence - one reason for pile supports. This begs the question of what kind of soil and moisture will be present. If it is well-drained sand, in arid conditions, less cover might be fine. If this is a coastal or other wet/salty environment, more cover or other protection might be needed.
If the soil is clay, is it wise to cast against soil without void forms? If the geotech includes assessment of PVR in the report for the soil, you may need voids.
There is latitude for engineering judgment here, and part of that is determining how much protection from corrosion is sufficient.
