Add third number 'c' to addition calculation

## Summary
Currently, the HelloWorld program only adds two numbers (`a = 123` and `b = 456`). We need to extend this to include a third number for a more comprehensive addition demonstration.

## Current Behavior
```java
Integer a = 123;
Integer b = 456;
System.out.println(a + b); // Outputs: 579
```

## Requested Enhancement
Add a third variable `c` with value `789` and modify the calculation to add all three numbers.

## Expected Implementation
```java
Integer a = 123;
Integer b = 456;
Integer c = 789; // New variable
System.out.println(a + b + c); // Should output: 1368
```

## Expected Output
The program should display `1368` (123 + 456 + 789) instead of the current `579`.

## Acceptance Criteria
- [ ] Add variable `c` with value `789`
- [ ] Update all print statements to include `c` in the calculation
- [ ] Verify output shows `1368` five times
- [ ] Maintain existing code structure and style

## Related
- Part of example-ticket-1 improvements
- Related to current HelloWorld.java functionality