There are only 2 Memories in Sequence 3.
Memory 1. Somewhere That's Green
- simply race your sibling to a marked location. Beat your sibling there to get full sync. You will meet someone named Henry Green.
Between Memory 1 and Memory 2, you must finish 5 missions in the Whitechapel borough:
1. Bounty Hunt (given by Sergeant Abberline)
2. Child Liberation (given by Clara O'Dea)
3. Gang Stronghold (kill all gang members)
4. Templar Hunt (given by Henry Green)
5. Gang War (appears after previous 4 are done)
Whitechapel is the first and easiest borough, so this should be easy to complete. The above missions 1 through 4 can be done in any order.
Memory 2. Freedom of the Press
- this memory can start when you travel to the Lambeth borough and reach the marked location, a gold B icon. You will have to do a few missions for your new friend. When you're done, visit him again. After a couple cutscenes, achievement will pop.