$\begin{array}{r}3 \frac{7}{8} \\ -\frac{3}{8} \\ \hline\end{array}$

Step 1 ：Convert the mixed number 3 7/8 to an improper fraction. The denominator remains 8, and the numerator is calculated by multiplying the whole number part by the denominator and then adding the numerator of the fractional part. So, the numerator is \(3 \times 8 + 7 = 31\). Therefore, 3 7/8 as an improper fraction is \(\frac{31}{8}\).

Step 2 ：Subtract the fraction \(\frac{3}{8}\) from the improper fraction \(\frac{31}{8}\). Since the denominators are the same, we can subtract the numerators directly. So, the result is \(\frac{31 - 3}{8} = \frac{28}{8}\).

Step 3 ：Simplify the fraction \(\frac{28}{8}\). Both the numerator and the denominator can be divided by 4, so the simplified fraction is \(\frac{7}{2}\).

Step 4 ：Convert the improper fraction \(\frac{7}{2}\) back to a mixed number. The whole number part is the quotient of the division of the numerator by the denominator, and the fractional part is the remainder of the division. So, the mixed number is 3 1/2.

Step 5 ：Final Answer: \(\boxed{3 \frac{1}{2}}\)