How many bags of top soil will I need to fill my flower garden of 25ft long 6inch deep and 3ft wide?

You'll need 50 bags. The math - LxWxD/bag volume: 75ft x 3f t x 0.5ft = 37.5 cubic feet of volume. Each 40lb bag is 0.75 ft^3. So, dividing you garden volume by the bag volume yields the number of bags you need.

You'll need 37.5 cubic feet (length x width x depth = volume). Bagged dirt comes in various volumes (usually 1, 2 or 3 cu ft), so the number you need depends on how large of a bag you buy.