How to take 3 elements at a time from an array and turn them into their own column vector?

I am working on a Hill-3 cipher encryption program, and so far I have the code to turn a user inputted string into an array C with each letter assigned a value from 0-25 (I'm not using any other symbols than the english alphabet). What I'm stuck on is how to convert this array C into a series of column vectors such that column vector 1 = [C(1);C(2);C(3)], column vector 2 = [C(4);C(5);C(6)], and so forth. Since the amount of elements in array C will vary, I need to loop this function somehow, and also need to add values to the extra rows of the last column vector if the amount of elements in C is not a multiple of 3. This is what I have so far (ps: I know there is probably a much more efficient way to do what I did with all the different cases, but that's not my issue lol):

prompt = 'Enter message for encryption:   ';
msg = input(prompt, 's');
msg = msg(~isspace(msg));
msg = lower(msg);
C = [];
for k = 1:length(msg)
    switch msg(k)
        case 'a'
            C = [C 0];
        case 'b'
            C = [C 1];
        case 'c'
            C = [C 2];
        case 'd'
            C = [C 3];
        case 'e'
            C = [C 4];
        case 'f'
            C = [C 5];
        case 'g'
            C = [C 6];
        case 'h'
            C = [C 7];
        case 'i'
            C = [C 8];
        case 'j'
            C = [C 9];
        case 'k'
            C = [C 10];
        case 'l'
            C = [C 11];
        case 'm'
            C = [C 12];
        case 'n'
            C = [C 13];
        case 'o'
            C = [C 14];
        case 'p'
            C = [C 15];
        case 'q'
            C = [C 16];
        case 'r'
            C = [C 17];
        case 's'
            C = [C 18];
        case 't'
            C = [C 19];
        case 'u'
            C = [C 20];
        case 'v'
            C = [C 21];
        case 'w'
            C = [C 22];
        case 'x'
            C = [C 23];
        case 'y'
            C = [C 24];
        case 'z'
            C = [C 25];
    end
end

Answer 1

[Andrei Bobrov]

msg = 'gfewqaxcvbhgf'; % example
[~,C] = ismember(msg,'a':'z');
C = C - 1;

out = reshape([C(:);zeros(mod(-numel(C),3),1)],3,[]);