Pretty sure that both of us just plotted f'' and eyeballed that the maximum was at x=0 but more formally

if you had taken the 3rd derivative you would find that it is non-zero on [0,3].

Thus the maximum has to be at a boundary point of your interval.

Clearly |f''(3)| < |f''(0)| so f''(0) must be the maxima on [0,3]

most likely a vertical asymptote within your limits of integration mean that your function isn't integrable on this interval.