The Tripods Trilogy

From Author Wars - https://authorwars.com/publications/the-tripods-trilogy-316668.html

Overview:

Title: The Tripods Trilogy
Author: John Christopher
Year: 1980
ISBN-10: 0020425708
Publisher: Macmillan
Price: $7.95
Binding: Paperback
Type: Omnibus
Title Reference: The White Mountains
Notes: Not an omnibus, but a boxed set of the three novels in the series.
Info from Locus #239 (November 1980). The ISBN and price is for the box set. It is likely that each book had its own ISBN and price.