Neim, 7 bytes

'π𝐂𝕋𝐬S𝕊

Explanation:

'π         Push 345
  𝐂        Get the characters
   𝕋       Vectorised multiply with the input
    𝐬       Sum the resulting list
     S𝕊    Subtract the input

Alternative program: 3𝐈ᛖ𝕋𝐬S𝕊

Instead of pushing 345 and then getting the characters, creates the array [1 2 3] using 3𝐈, then add 2 to each element with ᛖ.

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