If \(\frac{\mathrm{3} }{\mathrm{8}}\) of the combined wheat and white flour is wheat flour, then the remaining \(\frac{\mathrm{5} }{\mathrm{8}}\) must be white flour. Let's calculate the amount of white flour needed:
Total flour = 3 cups
Wheat flour = \(\frac{\mathrm{3} }{\mathrm{8}}\) × Total flour = \(\frac{\mathrm{3} }{\mathrm{8}}\) × 3 cups = \(\frac{\mathrm{9} }{\mathrm{8}}\) cups

Since the total flour is a combination of wheat and white flour, the amount of white flour is given by:
White flour = Total flour - Wheat flour = 3 cups - \(\frac{\mathrm{9} }{\mathrm{8}}\) cups

To compute the subtraction, we need a common denominator:
White flour = \(\frac{\mathrm{24} }{\mathrm{8}}\) - \(\frac{\mathrm{9} }{\mathrm{8}}\) = \(\frac{\mathrm{15} }{\mathrm{8}}\) cups

Therefore, you would need \(\frac{\mathrm{15} }{\mathrm{8}}\) cups (or \(1\frac{\mathrm{7} }{\mathrm{8}}\) cups) of white flour for the recipe.