Yes. 3 hours is normally fine to have time for a shower & bite to eat in domestic lounge before boarding. However, early am the showers are in high demand and weekday flights between main cities that depart between 0630 and 0800 in particular are usually completely full. If you have no checked bags, lounge access and are on a flexi fare for a domestic flight, then you could ask the agents in the lounge to be put on standby for an earlier full flight in case of no shows. Standby doesn't work well if you have checked bags as these would need to be transferred between flights.