After several are destroyed, one of the Aquila crew members will signal the three frigates on the horizon.There's good news and bad news regarding the three frigates: the Bad news is that only ONE of them will have an exposed powder store, meaning Connor needs to manually and physically expose the powder stores for the other frigates.
Aim the Swivel guns at the white reticle and FIRE when the reticle turns red to destroy it (1/2 - see screencaps above).
From here, Connor has to physically expose the powder stores of either of the remaining two frigates.
Since the upgraded Aquila is too powerful, ramming the other two frigate ships head-on will destroy them instantly.
The easiest way to expose the powder stores without using guns or cannons is to RAM the frigate ship's mast from the SIDE (basically, you'll t-bone it from the front).
By doing this, it won't destroy the frigate ship and Faulkner will yell, "Captain, there's an exposed side!
" signalling the ram attack was successful (see screencap above).
As soon as the exposed white reticle appears, quickly switch to Swivel guns and FIRE (2/2 - see screencap above).
Once that's done, destroy the rest of the ships and frigates to trigger the next objective.
DESTROY SHIPS USING ONLY ONE BROADSIDE: This objective requires that the Aquila destroy 3 or more small ships by firing only ONE shot.
Equip a Grape Shot when ready and sail in close to the nearby smaller ships.
When Connor is within range of several of the smaller ships, rotate the camera to ready the Grape Shot and then FIRE.