This is the 431st most frequent Lithuanian word.
trijų
"Trijų" means "of three" in English.
Here, "trijų" denotes the genitive form of the number three, used for indicating possession or association with the books.
Aš turiu trijų knygų rinkinį.
I have a set of three books.
In this sentence, "trijų" again in genitive case is used to quantify "žmonių" (people) in a construct expressing need.
Mums reikia trijų žmonių šiam darbui atlikti.
We need three people to complete this task.
"Trijų" describes "rūšių" (types) in the genitive case, as it is part of the structure indicating partitive or classification.