To solve for the value represented by the question mark in the equation \( \frac{1}{7} [?] = 3 \), we can rearrange the equation to isolate the question mark.
Starting with the equation:
\[ \frac{1}{7} [?] = 3 \]
We can multiply both sides by 7 to eliminate the fraction:
\[ [?] = 3 \times 7 \]
Calculating the right side gives:
\[ [?] = 21 \]
Thus, the answer is:
\[ [?] = 21 \]