11. (6 points) A sequence of numbers is arranged according to the following rule: $1,2,3,2,3,4,3,4,5,4,5,6, \cdots$ Starting from the first number on the left, the sum of the first 99 numbers is $\qquad$
【Solution】Solution: Consider every three numbers as a group, then the sum of each group is: $6,9,12,15 \cdots$
$$
\begin{array}{l}
99 \div 3=33 \text { (groups); } \\
S_{n}=n a_{1}+n(n-1) d \div 2 \\
=6 \times 33+33 \times(33-1) \times 3 \div 2, \\
=198+33 \times 32 \times 3 \div 2, \\
=198+1584, \\
=1782
\end{array}
$$

Answer: The sum of 99 numbers is 1782.
Therefore, the answer is: 1782.