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
You would need 31.88 cubic yards of topsoil to cover 855 sq feet at 6 inches deep.
.617 Yards
259.2593 sq. yards.
16 sq ft*0.5 ft = 8 cubic feet.
3,456 cubic yd.