3. (6 points) First, arrange the natural numbers starting from 1 in a sequence:
$123456789101112131415 \cdots$
Then group them according to a certain rule:
$1,23,456,7891,01112,131415, \cdots$
In the grouped numbers, there is a ten-digit number, which is
$$
2829303132 .
$$

【Solution】Solution: Method one:
According to the grouping rule, from 131415 onwards, it is 1617181, 92021222, 324252627, 2829303132 (ten-digit number), $\cdots$;

Method two: Before the digit, there should be $1+2+3+\cdots+9=45$ digits. Single-digit numbers have 9 digits, $10-19$ have 20 digits, $20-27$ have 16 digits, so the beginning of the ten-digit number should be 28, which is 2829303132. Therefore, the answer is: 2829303132.