Andrés - even though each iteration records only one second of audio data, it may be that each recording is of a different length (i.e. the audio array). In that case, your audios variable could be a cell array which will allow you to contain arrays of different lengths. Initialize it as
audios = cell(3,1);
and then initialize the kth element as
audios{k} = audio;
(I use k as the iterator since MATLAB treats i and j as representations for the imaginary number.)
To write each audio block to a different file, just do the following
filename = sprintf('output%04d.wav',k);
which will create file names such as
output0001.wav
output0002.wav
output0003.wav
etc.
Then write to file as
audiowrite(filename,audios{k},8000);
Try integrating the above into your code and see what happens.
