The cleanest (and probably fastest) method is to let load(filename) store into a variable, and let the function return this variable. So like this
function myData = checkfile(filename, pathname)
if isequal(filename,0)
warndlg('File not selected.', 'Warning!', 'modal');
myData = [];
else
cd(pathname);
myData = load(filename); % load variables
end
end
Note that I also added myData = [] for the case that there was no file selected. Otherwise your function would fail in this case.
myData will be a structure containing all the variables of the file. So in your case it will contain myData.a, myData.b and myData.c.
