Learning how to get random in c#?

Question

- Add the Console Application and name RandomStringPro
- Write the  method to return string which contains about 100 number characters from 0 to 9.
- Write the method to print out 3 number that their sum is their multiple.
- Example: Input: 1233474414325467321244435213 Output: 123 321 213.
Can you help me? Thank's