Problem 42854. Crunch that matrix!

You are given an M x N matrix. Write a script that will compress this matrix into an M x (N/3) matrix, where each of the terms in the new matrix are the sum of three consecutive terms on each row. You can assume that N is evenly divisible by 3.

For example, if your original matrix is:

[1     7    13    19    25    31
 2     8    14    20    26    32
 3     9    15    21    27    33
 4    10    16    22    28    34
 5    11    17    23    29    35
 6    12    18    24    30    36]

the output should be

 [1+ 7+13   19+25+31
  2+ 8+14   20+26+32
  3+ 9+15   21+27+33
  4+10+16   22+28+34
  5+11+17   23+29+35
  6+12+18   24+30+36]

or

   [21    75
    24    78
    27    81
    30    84
    33    87
    36    90]

Good luck!