It means only use one side of your ship to fire your cannons, it does not mean sink 3 ships in one volley.
The standard definition of broadside refers to firing the guns, and would mean that it would indeed need to be a single salvo. Now if you managed to do it in multiple salvo's, then that means that the developers screwed up the message.
Either way, grape shot, and a mission with large numbers of gunboats will take care of it. I managed to get five in a single salvo. A sixth one only survived because of extreme distance. So I turned into it at full sail and rammed it.
There are 10 types of people in the world. Those that know binary, and those that don't. Gamertag: Talon Omega