Separating array to individual rows

Hello.

I have an array called "Mat". In every row, there are 3 vectors with same length, one after another. From the last element of the third vector till the end of a row of "Mat", which has 5000 elements, there are "NaN" elements. There are 500 rows in a "Mat" for that combination. After that, the same is repeated two times more. Thus, the end size of a "Mat" is [1500,5000].

You can see how "Mat" is created in a code below. Values of each element here are just for example.

I would like to access to the first vector of each row. It goes from 1:16400, but with different number of elements (1000, 200 or 500). I made this with a for loop. It works, but I am interested, if the same could be achieved without any loop, just using vectorization.

You can better understand what I want, if you look to the code below.

%%Making a matrix Mat

A = [linspace(1,16400,1000) 4000*sin(1:1000) 3800*cos(1:1000)];
B = [linspace(1,16400,200) 4000*sin(1:200) 3800*cos(1:200)];
C = [linspace(1,16400,500) 4000*sin(1:500) 3800*cos(1:500)];

A = [A NaN(1,5000-length(A))];
B = [B NaN(1,5000-length(B))];
C = [C NaN(1,5000-length(C))];

Mat1 = repmat(A,[500 1]);
Mat2 = repmat(B,[500 1]);
Mat3 = repmat(C,[500 1]);

Mat = [Mat1; Mat2; Mat3];
Y = cell(size(Mat,1),1);

for i=1:size(Mat,1)
  Y{i} = Mat(i,1:(min(find(isnan(Mat(i,:))))-1)/3);
end

k = Y{1};
p = Y{750};
n = Y{1200};