If you mean the derivative of f(x) = 3 - e^(-x) is positive for all then then,
f'(x) = -e^(-x) * (-1)
= e^(-x)
As e^(-x) is greater than zero for all x in its domain, the f'(x) > 0 for all x.
Part 2: As x becomes very large (x-> infinity) e^(-x) becomes very small and hence the limit as x goes to infinity is f(x) = 3.