Read the question again:
It clear says "accepts an integer(n)" and "computes the value n + nn + nnn"
I.e. Read a value from the user (for example 72) and work out the value of:
72 + 7272 + 727272 = 734616
You aren't doing that: you are trying to read three values.
This is your homework, so I'll give you no code. But it's pretty simple:
1) Read a value from the user into a string -> n
2) Create a new string nn and append the original string twice
3) Create a new string nnn and append the original string three times
4) Convert each of these strings to a numeric value: check your notes and you'll find information of either Convert.ToInt32 (basic and bad) or int.TryParse (much better, but harder to use). put them in three new integer variables iN, iNN, and iNNN.
5) Add together iN, iNN, and iNNN, then print the result.