No. Consider the functions given below :
Let f and g be two functions from the set of real numbers to the set of real numbers defined by
f(x) = x , g(x) = x and h(x) = - x
Clearly, inverses of these functions are functions.
But, f(x) + h(x) = x + ( - x) = 0 does not have inverse.
Also, f(x) - g(x) = 0 does not have inverse.