Here is a list of the memory's in Sequence Three, and their full syncronisation requirements.
SEQUENCE THREE - THE FIGHTER THE LOVER AND THE THIEF
Memory 1 - Between a Rock and a Hard Place
Full sync: Kill only the Borgia Captain.
Tip: The Borgia Captain will be directly in front of you when you begin. He will run as soon as he sees you, be careful not to accidentally kill someone else while chasing the captain. At the tower, approaching from the water will have you at the backs of your enemies - they will not notice.
Memory 2 - High Stakes Negotiation
Full sync: Don't swim.
Tip: There's really only one place where there is a danger of you falling in the water and that's when fighting the slavers on the boat. When I fought them I just played the defensive and waited for them to attack me.
Memory 3 - Double Agent
Full sync: Don't be detected while reducing notoriety. Remain undetected until returning to La Volpe.
Tip: I usually go for posters when reducing notoriety and try to stick to the rooftops, there are less guards up there but also hanging out with the crowds is advisable and blend whenever possible.
This is a very easy memory sequence to 100%. If you havn't gotten the "Perfect Recall" achievement yet, this is the better one to go for it with. Screw anyone who says sequence two. :P