3. (3 points) If $a$ represents a three-digit number, and $b$ represents a two-digit number, then, the minimum value of $a+b$ is $\qquad$ the maximum value of $a+b$ is $\qquad$ , the minimum value of $a-b$ is $\qquad$ , and the maximum value of $a-b$ is $\qquad$ .
【Analysis】The larger the two numbers, the larger their sum; the smaller the two numbers, the smaller their sum. The closer the two numbers are, the smaller their difference; conversely, the larger their difference. Therefore, according to the conditions, find the largest and smallest three-digit and two-digit numbers, and calculate accordingly.

【Solution】The smallest $a+b$ is $10+100=110$,
The largest $a+b$ is $99+999=1098$,
The smallest $a-b$ is $100-99=1$,
The largest $a-b$ is $999-10=989$.
Therefore, the answers are: $110,1098,1,989$.