3. (8 points) As shown in the figure, a multiplication vertical calculation has already filled in $2, 0, 1, 6$. What is the product? 

Translate the text above into English, please keep the original text's line breaks and format, and output the translation result directly.
6156
【Solution】According to the problem, 
the three-digit multiplier multiplied by 2 results in a four-digit number, so the hundreds digit must be greater than 4. Given that the result is a number in the 1000s, the hundreds digit of the multiplier is 5. Additionally, the tens digit of the three-digit multiplier multiplied by 2 does not carry over.

At the same time, the three-digit number multiplied by a number still results in a three-digit number, which implies that the digit before 2 in the multiplier is 1, making the two-digit part of the multiplier 12.

Based on the last digit of the result being 6, the units digit of the three-digit multiplier must be 3.
Given the digit 1, $0+1=1$, so the three-digit multiplier is 513.
\begin{tabular}{rrrr} 
& 5 & 1 & 3 \\
$\times$ & & 1 & 2 \\
\hline 1 & 0 & 2 & 6 \\
\hline 5 & 1 & 3 & \\
\hline 6 & 1 & 5 & 6 \\
\hline
\end{tabular}

Therefore, the answer is: 6156