How it works

DµL‘:3x2jSạ¥¥LRṁ@ḅ1E  Main link. Argument: n

D                     Decimal; convert n to base 10, yielding a digits array A.
 µ                    Begin a new chain with argument A.
  L                   Compute the length of A.
   ‘                  Increment; add 1 to the length.
    :3                Divide the result by 3.
                      This yields the lengths of the outer chunks.
      x2              Repeat the result twice, creating an array C.
             L        Compute l, the length of A.
            ¥         Combine the two links to the left into a dyadic chain.
                      This chain will be called with arguments C and l. 
           ¥              Combine the two links to the left into a dyadic chain.
         S                    Take the sum of C.
          ạ                   Compute the absolute difference of the sum and l.
        j                 Join C, using the result to the right as separator.
                      We now have an array of the lengths of all three chunks the
                      digits of n have to be split in.
              R       Range; map each chunk length k to [1, ..., k].
               ṁ@     Mold swapped; takes the elements of A and give them the shape
                      of the array to the right, splitting A into chunks of the
                      computed lengths.
                 ḅ1   Convert each chunk from unary to integer, computing the sum
                      of its elements.
                   E  Test if the resulting sums are all equal.