how to combine matrix easier

Hello,
I want. How can i do this (below code block) with a loop? 
I have to do this with a loop because, i will be doing this with a lots of a matrices. please help me. 
Thank you.

a1=[1,2,3; 4,5,6; 7,8,9]
a2=[10,11,12; 13,14,15; 16,17,18]
a3=[19,20,21; 22,23,24; 25,26,27]

b1=[a1(:,1);a1(:,2);a1(:,3)] 
b2=[a2(:,1);a2(:,2);a2(:,3)] 
b3=[a3(:,1);a3(:,2);a3(:,3)] 
C=[b1,b2,b3] 

[BobH]

You are reading a1/a2/a3 in column order, all rows of col 1, then all rows of col 2, then all rows of col 3. This is the same as the single-index linear indexing

b1 = a1(:)

All the b1/b2/b3 assignments can be eliminated

C = [a1(:) a2(:) a3(:)]

[Ameer Hamza]

BobH, the actual problem is that OP wants to do it with lots of matrices, so manually concatenating array will not be practical.

Answer 1

[Ameer Hamza]

If the data is stored properly, i.e., in cell array instead of using separate variables, the following one-liner will work

a{1}=[1,2,3; 4,5,6; 7,8,9];
a{2}=[10,11,12; 13,14,15; 16,17,18];
a{3}=[19,20,21; 22,23,24; 25,26,27];
 
C = cell2mat(cellfun(@(x) x(:)', a, 'UniformOutput', 0)')';

[metumetu]

Thanks for your answer, it works now. But i have two question.

1)Can we do this with a for loop?

2)My data ("a" matrices) is lots of txt files. I copy all of them and paste to create each "a" matrices a1, a2, a3 and so... how can i do that more practical? Thank you.

[Ameer Hamza]

For first question: yes, we can do it with for loop

a{1}=[1,2,3; 4,5,6; 7,8,9];
a{2}=[10,11,12; 13,14,15; 16,17,18];
a{3}=[19,20,21; 22,23,24; 25,26,27];

C = zeros(numel(a{1}), numel(a));
for i =1:numel(a)
    C(:, i) = a{i}(:);
end

There is no need to copy and paste values. For reading txt multiple txt files automatially, refer to my answer on this question: https://www.mathworks.com/matlabcentral/answers/509790-read-and-manipulate-multiple-csv-files

[metumetu]

Thank you so much for your help. Thats great for me

[BobH]

Another recent post that involved reading from text files

https://www.mathworks.com/matlabcentral/answers/510374-importing-txt-files-and-using-loops