12 sq yds
Three acres three inches deep is 403.33 cubic yards.
Nor sure how pee gravel is packed, but you need enough for 64 cubic feet.
Your pool's volume is 40ft x 16ft x 5.75ft = 3680 cubic feet. 1 cu.ft. = 1728 cubic inches 1 US gallon is 231 cubic inches 3680 x 1728 / 231 = 27528.312 US gallons in that pool.
If it is 4 inches deep/thick, 2 cubic yards. If it is 6 inches deep/thick, 3 cubic yards.
6 inches = 1/6 yards. So 70 sq yards * 6 inches = 70*1/6 cubic yards = 11.66... cubic yards.
40.1 cubic yards
2.2222222 cu. yards
62/3 cubic yards for every 1-foot deep. 5 cubic yards for every 9-inches deep. 31/3 cubic yards for every 6-inches deep. 5/9 cubic yard for every 1-inch deep.
If you say 2 inches deep, I assume you really mean cubic yards, as in how much you would have to buy? If so, the answer is 7.77, so you would need about 8 yards.
.617 Yards
259.2593 sq. yards.
16 sq ft*0.5 ft = 8 cubic feet.