3. (10 points) In a $6 \times 6$ grid, place rectangles, each covering exactly 2 squares. If no two rectangles share a common edge (they can share a common vertex), the maximum sum of the numbers in the squares covered by the rectangles is ( )
A. 266
B. 304
C. 342
D. 380
【Analysis】This problem tests the concepts of maximum and minimum.
【Solution】Solution: Since there are no common sides between any two rectangles, each rectangle covers the numbers 20 and 18, with an average of 19,

thus the sum of all numbers is $36 \div 2 \times 19=342$.
Therefore, the answer is: $C$.