2 added 329 characters in body
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Pyth, 7 bytes

jc3.[9d

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Explanation:

jc3.[9d Expects quoted input.
  3     3
     9  9
      d ' '
        Q (eval'd input) as implicit argument
   .[   Pad B on both sides with C until its length is a multiple of A
 c      Split B to chunks of length A, last chunk may be shorter
j       Join A on newlines

Pyth, 7 bytes

jc3.[9d

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Pyth, 7 bytes

jc3.[9d

Try this online.

Explanation:

jc3.[9d Expects quoted input.
  3     3
     9  9
      d ' '
        Q (eval'd input) as implicit argument
   .[   Pad B on both sides with C until its length is a multiple of A
 c      Split B to chunks of length A, last chunk may be shorter
j       Join A on newlines
1
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Pyth, 7 bytes

jc3.[9d

Try this online.