Hard to say exactly but you can get a rough estimate with the information you gave by assuming that it's a constant downsloping grade from one end of the pool to the other. Using that, then you'll have roughly 15700 gallons.
Assuming it is filled to the top, an 18 foot diameter, 3 foot deep pool holds 5,711 gallons. Take the radius in feet (half the diameter), square it, multiply by pi (3.1416) to get area in square feet. Area times depth in feet gives you cubic feet. Cubic feet times 7.481 (gallons per cubic foot) gives you gallons. So: 9^2 (81) times 3.1416 times 3 times 7.481 = 5711.06
That is not enough information to tell you how many gallons are in the pool.
The length/width of the deep/shallow end individually are needed for an approximation.
I can give you the min and max gallons of the pool though.
A gallon is 231/1728 cu. ft.
Assuming the whole pool was 5 ft deep: 18 ft X 38 ft X 5 ft = 3420 cu. ft = 457.1875 gallons maximum.
Assuming the whole pool was 3 ft deep: 18 X 38 ft X 3 ft = 2052 cu. ft = 274.3125 gallons minimum.
I think you would also need the distance where the slope up to the shallow end starts.
10,408 gallons.
7,646.4 gallons are needed.
About 24 gallons.
About 18,402 gallons.
Approximately 1,616 US gallons.
1.5x4x6x7.48 gallons/cubic foot=269.28 us gallons
without knowing the actual lengths of the shallow and deep ends, nothing more than a very loose ballpark number is possible.
A 15-foot x 42-foot oval pool with an 8-foot deep end and a 3-foot shallow end will contain about 20,443 gallons of water.
13,594 US gallons of water.
If rectangular about 21,000 gallons if oval about 16,000 gallons
25000 gallons