Side mission 1
Defeat the officer to gain the information. Use fists to get rid of his guards and afterwards disarm the officer himself.
Approach him outside of combat to complete the mission.
Side mission 2
Your mission will be planting a falsified document to a designated informer. In order to do that, sneak up to him from behind and hold down the interaction button. The enemy is so slow with reacting, that you should make it even if he notices you.
Side mission 3
Stop the execution by killing all the marked soldiers.
Speak with your contact in the district, head to the shore and use the "Covert Escort" option from the Assassins Guild menu.
That way you will call in disguised recruits and will be able to pass by enemy patrols unnoticed.
Approach the designated target and kill him to end the mission.