The page below of our Assassin's Creed III Remastered guide contains a walkthrough for Third Energy Source.
After you reach the Abstergo headquarters, move onwards until you're attacked by a security group. Defeat them quickly and enter the elevator\.
When the lift stops, start climbing up. The path will you along both sides of the elevator shaft.
Upstairs there will be more enemies waiting for you. Kill them, enter the room on the left and keep heading forward while eliminating the attackers.
Eventually you will reach a corridor with a broken window where you will be attacked by Cross. After a short cutscene, start chasing him.
Inside the big room, try to move constantly so that the guards cannot hit you.
Your task is to kill the running Templar.
After you manage to do it, you will reach the fifth floor where Vidic is hiding. To find him, firstly turn left and afterwards run straight all the way, killing all enemies on your way.
After the last of them is dead, the secretary will open the door to the boss' office.
During the conversation with Vidic, press the attack button to activate the apple.
In order to leave the building, you will once again have to press the button and afterwards head forward all the way.
As you reach the cave once again, go up the stairs on the right and move onwards, jumping over the holes in the ruins.
After the first animation with Juno you will have to climb up to the upper floor, jumping between orange crystals.
Upstairs, turn left and jump onto two pipes. Using them you will reach a wall full of crystals.
Climb it even higher and follow the orange crystals.
That way you will reach the spot where Desmond will use the power source.
After a short conversation you will be able to return to the Animus at once or speak with the team members beforehand.