3. Divide the diagonal of rectangle $\mathrm{ABCD}$ into 12 equal segments, and connect them as shown in the right figure. The total area of the blank parts inside rectangle $\mathrm{ABCD}$ is 10 square centimeters. Therefore, the total area of the shaded parts is ( ) square centimeters.
(A) 14
(B) 16
(C) 18
(D) 20
Analysis: As shown in the figure, the large rectangle is divided into six smaller rectangles. According to the similar model, the length-to-width ratios of these smaller rectangles are sequentially $1: 2: 3: 4: 5: 6$. The area ratio of the blank part to the shaded part is: $\left[1^{2}+\left(3^{2}-2^{2}\right)+\right.$ $\left.\left(5^{2}-4^{2}\right)\right]:\left[\left(2^{2}-1^{2}\right)+\left(4^{2}-3^{2}\right)+\left(6^{2}-5^{2}\right)\right]=15: 21=5: 7$, so the total area of the shaded part is $10 \div 5 \times 7=14 \mathrm{~cm}^{2}$. The answer is A.