Selecting an entry of a row in a large matrix

Can any one help me with this ?

I want to select each 3rd row and first entry of it and want to output all values in a matrix. Next I want to remove this out put vector from original matrix a =

    64     2     3    61    60     6     7    57
     9    55    54    12    13    51    50    16
    17    47    46    20    21    43    42    24
    40    26    27    37    36    30    31    33
    32    34    35    29    28    38    39    25
    41    23    22    44    45    19    18    48
    49    15    14    52    53    11    10    56
     8    58    59     5     4    62    63     1
ans

b =[17 41];% this is what I need in first step

and then final matrix with removing every 3rd row (which omits the every third row)

Thanks, Amb

Answer 1

[amberly hadden]

out = a(3:3:end,:)

this worked but now I just wanna remove these rows from original matrix

Answer 2

[James Tursa]

b = a(3:3:end,1); % 1st element of every 3rd row
a(3:3:end,:) = []; % remove every 3rd row

[amberly hadden]

Thank you so much tursa