What is the natural frequency of a 3rd order system with 3 real roots?
If a system only has real roots the response is comprised of exponential terms and not oscillatory terms. In this case

$$y(t) = C_1e^{-4t} + C_2e^{-5t} + C_3e^{-3t},$$

thus there is no such thing as a natural frequency of this system because there are no oscillating terms in the solution.