You are creating variables. You need to modify your syntax to use the xlabel and ylabel functions.
Also, labels apply to the whole axes.
z = [1 4 7; 2 5 8; 3 6 9;];
bar3(z)
xlabel(['a' 'b' 'c'])
ylabel(['A' 'B' 'C'])
I think you are actually trying to modify the xticklabels and yticklabels. You also need to change your syntax slightly to do that.
z = [1 4 7; 2 5 8; 3 6 9;];
bar3(z)
xticklabels({'a' 'b' 'c'})
yticklabels({'A' 'B' 'C'})
