Using ever increasingly powerful nitration reagents. Mononitro goes on without concentrated acids, IIRC, but by trinitro you are using fuming nitric or sulfuric acids.
Other heavily deactivated compounds (ex dichlorbenzene) can only be trinitrated under conditions with hot, fuming sulfuric acid. For even more deactivated compounds, exotic reagents like nitronium tetrafluoroborate or dinitrogen pentoxide are used.