This question involves understanding changes to the digits of two-digit numbers. Let’s break it down step by step:

1. Represent a two-digit number as $10a + b$, where $a$ is the tens digit and $b$ is the units digit.
2. If the units digit $b$ is decreased by 3, the new number becomes: $10a + (b - 3)$
3. If the tens digit $a$ is increased by 1, the new number becomes: $10(a + 1) + (b - 3)$ This simplifies to: $10a + 10 + b - 3 = 10a + b + 7$
4. Comparing the original number ($10a + b$) and the new number ($10a + b + 7$), the increase for each number is $7$.
5. For three two-digit numbers, the total increase is: $3 \times 7 = 21$

Thus, the total increase in the sum of these three numbers is 21.

The correct answer is: C) 21.

Would you like further explanation or details?

Here are 5 related questions to explore:

1. How can changes to digits affect the overall value of a number?
2. What is the general formula for representing a two-digit number?
3. How can you apply similar logic to three-digit numbers?
4. What is the impact of increasing or decreasing a digit by a specific amount in other base systems?
5. How does this problem change if the operations are reversed (e.g., units digit increases and tens digit decreases)?

Tip: When working with digit problems, clearly define the position and represent numbers using place values like $10a + b$ for systematic analysis.