3. Let the set $M \subset\{1,2, \cdots, 2011\}$ satisfy: any three elements in $M$, one is a multiple of another. Find the maximum value of $|M|$.

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Hint: First consider a $|M|-1$ element set, consisting of numbers that are powers of 2 or 3 times a power of 2; then consider the interval $[n, 2 n-1]$ and determine the maximum number of elements from $M$ it can contain. Answer: 21.