You have killed Silas and freed the captives, which marked the end of Sequence 2. For next sequence, you will have to leave Boston and travel to Frontier for obvious reasons.
Assassin’s Creed 3 – Sequence 3 100% Sync
Mission 1: Unconvinced
For this mission, you have to leave Boston and travel to the Frontier. It is quite a distance away so I would suggest taking a horse.
Travel to the checkpoint to find Charles waiting with two horses. Mount the horse and follow Lee as he takes you to the camp site of the mysterious woman. Once there, follow her tracks to her new location.
After the cut scene, chase her through the woods until you come to a camp where you will be attacked by wolves. Kill them by performing the quick time events that occur. Make sure not to lose more than half your health throughout the mission to complete the side quest.
Once the wolves are dead, look up at the trees surrounding the camp to spot a wooden platform where she is hiding. When you approach this tree, she will take off once more, and you will have to chase her again.
When you finally catch up to her, there is a cut scene in which you both agree to kill General Braddock together. Go to the Wright Tavern indicated on your mini map.
Once inside you will have to eavesdrop for clues. Make sure to blend in so as not to raise suspicion while eavesdropping. Once you have eavesdropped on both the conversations, make you a way out of the tavern.
You will be stopped on the way by guards trying to pick a fight. When the fight begins you receive a side quest to use breakable objects at least twice in this fight. You can do this by using counter attacks while standing near breakable objects such as tables and chairs.
Mission 2: Execution is Everything
You will have to travel quite a distance to the next checkpoint so take a horse. At the checkpoint, Ziio will be waiting for you; your mission will be to infiltrate St. Mathieu fort nearby. Look out for a cart on your way and climb on the rear.
Once inside the fort you will get two new objectives. The first is not to kill any soldiers throughout the mission, which simply means you have to sneak around and be careful not to get caught. The other main quest is to sabotage the two enemy cannons in this area.
Get out of the cart and hide in the bush nearby. You will have to eavesdrop nearby guards. Follow the mini-map to their position, hide in the bushes nearby and wait for the opportune time when the guards aren’t looking to climb up the building and follow their conversation. Don’t worry, you won’t have to climb down the building, just stay near the edges.
Your new objective is to get the map found in the tent to the south, jump into the bushes on the way and lean against the closest tent. When the guard is coming your way, whistle to keep him distracted and rush through the bushes at the back into the tent and grab the map.
Immediately make your way into the bushes past the tent before the guard comes back. You can find one of the cannons past the bushes further to the south. Make sure nobody is looking and run towards the cannon, sabotage it and rush back to the bushes.
Jump down the stairs to the east and go further down the trench avoiding being detected. The other cannon can be found to the south.
Be careful of the guards’ patrolling here. The guard near the cannon stops at the cannon for a bit and goes back so time your movement accordingly. After sabotaging the cannon, jump down into the water below and swim to the right keeping near the cliff, and you will end up back to Ziio. Talk to her to end the mission.
Mission 3: The Braddock Expedition
Head on towards the checkpoint to start the mission. After the cut scene, you get two new objectives. The first is to kill two guards without triggering open conflict. This can be tricky, try to distract them and kill them when they come to investigate to make things easy.
You can also try to hide in the bushes and execute the kill while they walk by on their patrol. The other objective is to destroy three powder carts.
Mount the horse and ride towards Braddock. A new sequence will start in which you have to chase Braddock on the horse. Look out for the powder carts on the way.
Shoot them by highlighting them and pressing y. They will appear near the beginning, middle and end. Once the objective is completed, chase down Braddock to complete the mission.