What you are trying to do is not recommended in MATLAB since dynamically creating variables is not useful. You have several other options about creating variables. I suggest you to read the following tutorial:
For your problem, following code should do it:
for i=1:3
for j=1:4
for k=1:5
eval(sprintf('a_%d%d%d=2*%d*%d*%d',i,j,k,i,j,k));
end
end
end
Please read the tutorial in order to understand what I meant.
