If you really want to use inconvenient nested cell arrays, this will work with your example data:
b = [a{:}];
b = [b{3,:}]
Note that using just one cell array (no nested cell arrays) makes this task simpler:
a = cell(10,3);
for n = 1:10
a{n,1} = datestr( now() );
a{n,2} = now();
a{n,3} = n;
end
a{1,1}
a{1,2}
b = [a{:,3}]
