Another Slytherin victory! That means the answer must be...
In His Hollow Eyes ends with Hermione raising her wand to kill Harry, and the other two are rather obvious
All three protagonists are facing the death of someone they loved.
As of this moment in time, all three have a review count that is a multiple of 2. Plus, they go up in doubles, so
A Lightness - 2
Through the Veil - 8
Wow, you guys are good. I will have to think of something else.
All three stories involve unsuccessful relationships:
The Muggle mother of Remus Lupin leaves his father.
Hermione declines Viktor's romantic overtures.
Eileen Prince has an unhappy marriage with Tobias Snape.
All three stories mention some historical background:
The history of the plague village where the Lupin family lives.
The reference to Grindelwald.
The reference to the Brothers Grimm and the magical historical basis of fairy tales.
(be prepared for some extremely tenuous suggestions)
Each story features an unexpected visitor:
Dumbledore visiting the Lupins
Krum visiting Hermione
Prince Phillip visiting Aurora
All three feature forests:
The Lupin cottage is in a forest
Hermione and Krum walk towards the Forbidden Forest
Phillip and Aurora meet in a forest
All three feature magical creatures:
Fawkes is mentioned as being in Dumbledore's office
House-Elfs prepare Hermione's breakfast (and Hagrid mentions the giant spiders)
A House-Elf tells Phillip that Aurora has fainted
...that's enough wild speculation for now, I think