You cannot do that in general.
In the special case that all the categories are a single character long, then you can do it:
m = 4;
% creating a random m-by-3 categorical array
C = categorical(num2cell(char(64+randi(26,[m 3]))))
% converting C to an m-by-3 char
reshape(char(C),m,[])
But if any of the categories is more than one character, that's not going to work because you have more than 3*m characters in total.
Instead, convert to a string array:
string(C) % m-by-3 string array
Or a cell array of character vectors:
cellstr(C) % m-by-3 cell array
