Actually it was very commun for warships to have cannons the stern and prown.
they were important for chases between ships,
As for the stern most battleships had two cannons located under the cabin/balcony and/or up to 4 small cannons on the highest deck which were in most cases swivel guns mounted on the ship.
(the higher a cannon is located on a ship, the smaller it usually is. Logical because it's best to keep the weight as low as possible to sail balanced and avoid capzizing the ship)
As for the prow; galleons and other ship types featuring forecastles often had a pair of cannons in it, other battleships without forecastle had cannons on the deck closest to the prow.
These cannons where long and more accurate than the regular broadside cannon.
A Triple cannon as seen on the flying dutchman in POTC didn't excist, atleast there's no evidence of that.
Keep in mind though that those "chaser' cannons were present on warships/battleships mostly and were considered an addition to the arnament rather than main weaponry.
An armed trading ship rarely had such features.
They're very popular among ship builders, maybe because of the POTC movies :p
A great movie about warships you should definitely see is "Master and Commander: the far side of the world"
(with Russell Crowe)
It's very realistic considering ships sailing and battles. Permalink