Final answer:
If a fire breaks out in #3 hold at sea, you should call the Master, shut down ventilation, and sound the alarm, but not inspect the fire yourself.
Step-by-step explanation:
When a fire breaks out at sea on a vessel, particularly in #3 hold just forward of the bridge at night, you would NOT proceed to the space and inspect the extent of the fire. Instead, your immediate actions should include calling the Master, shutting down the cargo hold ventilation to prevent the spread of the fire, and sounding the fire alarm signal to roust out all hands for emergency response. Entering the space to inspect the fire without proper equipment and backup is extremely dangerous and contrary to standard safety procedures on a ship.