Poseidon arises with his mighty trident to serve you trilogies at Amazon.com.
Nancy Springer
Sea King Trilogy: Madbond, Mindbond, and Godbond for $3.99.
Robert Evert
The Riddle in Stone Series for $1.99.
Mindy Klasky (a cheater who sneaks in five books)
The Glasswrights Series for $3.99.