You won't be able to do that with numeric operations. MATLAB will always return final answer as numeric value.
The closest you might get is using symobilc variable, which supports natural log -
syms f(x,y,z)
f(x,y,z)=log10(x)-log10(y)+log10(z);
val=simplify(f(64,128,32))
str=class(val)
