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.

Try it online!

Neim, 7 bytes

'π𝐂𝕋𝐬S𝕊

Explanation:

'π         Push 345
           The character ' pushes the next character's index in the codepage plus 100.
           The characters ", + and * do that same thing except add a different number.
           This means that in Neim, all 3 digit numbers can be expressed with 2 characters.
           This commit was pushed 8 days before the answer was posted.
  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 adds 2 to each element with ᛖ.

Try it online!

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

Try it online!

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.

Try it online!