How many gallons of water will it take to fill a tank measuring 8' long 28" wide and 3' deep

You're looking for the volume function here.  V=LWH

In this case the length is 8 feet
L=8
Width is 28 inches, and since there's twelve inches in a foot, it's 28/12 feet
W=28/12=2
Height, or in this case depth, is 3 feet
H=3

V=LWH
V=(8)(2)(3)
V=56ft

Now you know how many cubic feet are in your tank.  There are just around 7.5 gallons in a cubic foot.  An estimated answer then would be the volume times 7.5

56·7.5=420

There's an estimated answer.  If you need exact EXACT, then using the exact amounts and everything the answer comes out to 418.909 gallons.