Help! I forgot my password!

05AB1E, 9 bytes

5Ý3ãʒÇƶOQ

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Returns list of lists of digits.

C, 113 108 bytes

f(int h){int i=47,j,k;while(++i<54)for(j=47;++j<54)for(k=47;++k<54;)if(h==i+j+j+k*3)printf("%c%c%c",i,j,k);}

It is unique to see what is meant for output, the output is of the format: 200010

All passwords are written as 3-digits without delimiter.

Jelly, 16 bytes

Ṿ€Oæ.J
6Ḷṗ3Ç⁼¥Ðf

A monadic link returning a list of lists of digits.

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How?

Ṿ€Oæ.J - Link 1, hash: list of integers (the digits of a password)
Ṿ€     - unevaluate €ach (giving a list of characters)
  O    - cast to ordinals (Ṿ€O could actually be replaced with +48 too)
     J - range of length (i.e. [1,2,3] in all use-cases)
   æ.  - dot product

6Ḷṗ3Ç⁼¥Ðf - Main link: number, n
6Ḷ        - lowered range of 6 = [0,1,2,3,4,5]
  ṗ3      - Cartesian power with 3 = [[0,0,0],[0,0,1],...,[5,5,5]] (all passwords)
       Ðf - filter keep if:
      ¥   -   last two links as a dyad (right implicitly n):
    Ç     -     call last link (1) as a monad
     ⁼    -     equals right?