What each one actually is
Topsoil is the top few inches of the earth, scraped up, screened to remove rocks and roots, and sold by the yard or the bag. It is dark from decomposed organic matter, which is what plants feed on. Quality varies by source: one supplier's topsoil is sandy loam, another's is heavy clay.
Garden soil is not scraped from anywhere. It is a recipe: topsoil blended with compost, peat moss, bark fines, or manure. Bagged garden soil is designed to go straight into a planting hole or bed with no amendment. You pay more per cubic foot for that convenience.
Fill dirt is what is left after the topsoil is scraped off: subsoil, mostly clay and mineral matter, almost no organic content. It is the cheapest of the three by far, often free from construction sites. It compacts hard, drains poorly, and grows weeds better than it grows vegetables.
The price ladder
Prices vary by region, but the ordering is consistent everywhere. Fill dirt is the cheapest, often 10 to 25 dollars a yard or free from job sites. Screened topsoil runs roughly 30 to 60 dollars a yard. Garden soil is the premium product, and bagged garden soil can cost several times what bulk topsoil costs per cubic foot.
The price gap is why the layering trick matters. For a 12 inch deep bed, filling the whole thing with garden soil is expensive. Putting fill dirt on the bottom, plain topsoil in the middle, and a garden soil or compost blend in the top 6 to 8 inches where roots actually feed gives you the same result at a fraction of the cost. Roots of most vegetables live in the top 8 inches, so depth below that is just ballast.
Which one for lawns
Use screened topsoil for lawns, not garden soil and definitely not fill dirt. Lawns need a mineral base with moderate organic matter, and the price of garden soil buys you nothing extra for grass. For topdressing, choose screened topsoil free of large clods so it sifts through the turf. For low spots deeper than 2 inches, fill dirt is acceptable for the deep base layer with topsoil on top, since grass roots will live in the top layer.
One caution with lawns: cheap unscreened topsoil can carry weed seeds. The bark-heavy blends in bagged garden soil are worse, since bark ties up nitrogen as it breaks down and starves young grass. Stick with screened topsoil and the topsoil calculator to size the job.
Which one for vegetables
For vegetables, plain topsoil works but is incomplete. Amend every load with 2 to 3 inches of compost worked into the top 8 inches. That blend is roughly what bagged garden soil is, at a much lower price per yard. For raised beds shallower than 8 inches, where the blend is the whole bed, bagged garden soil is the simpler choice and the price premium buys you consistency.
Fill dirt has no place in a vegetable bed, ever. Its density suffocates roots and its lack of organic matter means you would be amending it for years before it behaves like soil. Saving 20 dollars a yard on the base of a vegetable bed is the kind of economy that costs you a season of crops.
Which one for grading and filling
Grading is the one job where fill dirt is the right answer. Raising a low corner of the yard, filling a hole, building up a slope: all of these are volume jobs where nutrients do not matter and compaction is the goal. Topsoil would work but wastes money, and it settles more than fill dirt does.
One rule for grading with fill dirt: cap the top 4 to 6 inches with topsoil if you plan to grow grass or plants over it. A lawn seeded directly on fill dirt comes up thin and yellow. The topsoil cap is the only part the plants ever see, so keep the expensive layer on top and the cheap layer underneath.
How to mix them in one project
Most yard projects use two of the three. A new garden bed on poor clay: fill dirt to raise the grade if needed, then 8 to 12 inches of topsoil blended with compost on top. A lawn renovation: screened topsoil throughout, with fill dirt only under the deep low spots. A raised bed: garden soil in the top half for instant planting, plain topsoil below.
Order the cheap layer first and the expensive layer second, since delivery trucks can usually drop two loads in one trip. And size each layer with the volume formula, length times width times depth divided by 27, so you order the right yards of the right material instead of one blended guess.