This is the 16013th most frequent Polish word.
trzem
"Trzem" is the dative case form of "trzy," meaning "to three."
The word 'trzem' is the dative form of the numeral 'trzy' and it is used here to indicate to whom the books are being given, describing that there are three individuals involved.
Daj książki trzem uczniom.
Give the books to three students.
In this sentence, 'trzem' refers to the number of friends being helped, again using the dative case to denote the indirect object of the help.
Pomogłem trzem przyjaciołom w przeprowadzce.
I helped three friends with their move.
Here, 'trzem' identifies the three specific participants being congratulated, utilizing the dative case in a context of addressing or recognizing individuals.