Here are some threesies puzzles. What do the three cards have in common? It won't be something most or many Dominion cards share - #2 isn't "they all cost $4" - but will not necessarily be unique to those three cards.
1. Apothecary, Followers, Tournament
2. Coppersmith, Mining Village, Smithy
3. Bureaucrat, Harem, Navigator
4. Bridge, Curse, Horn of Plenty
5. Ambassador, Bag of Gold, Young Witch
6. City, Followers, Walled Village
7. Black Market, Possession, Watchtower
8. Curse, Followers, Harem
9. Ambassador, Fairgrounds, Mountebank
10. Counting House, Goons, Nobles