A triad is a set of 3 skeins of fingering weight yarn that coordinate in color. They are intended to help you knit or crochet a shawl that will really shine. Each set contains at least 1200 yards. There are many, many shawls on Ravelry that call for this yardage. Each set is one of a kind. Here’s a VERY incomplete list:
Sea Swells, Dotted Rays, and Doodler by Stephen West
Free your fade and Woodland Shawl by Andrea Mowry
Joker & the Thief and Quicksilver and True Colors by Melanie Berg
In fact Ravelry says that there are over 4300 patterns that use fingering weight yarn in the range of 900-1200 yards.