Maybe use this
% Create sample data.
N = 100
M = randi(9, N, 3)
% Split matrix up into parts.
[rows, columns] = size(M)
endingRows = round([0.7, 0.85] * rows)
m1 = M(1:endingRows(1), :);
m2 = M(endingRows(1)+1:endingRows(2), :);
m3 = M(endingRows(2) + 1 : end, :);
If you want, you could use randperm to randomize where the rows are taken from.