This is the 17553rd most frequent Russian word.
трехзначный
Three-digit.
In this sentence, 'трехзначный' is an adjective describing a number that consists of three digits.
Это число является трехзначным.
This number is a three-digit number.
Here, 'трехзначных' is the genitive plural form of the adjective 'трехзначный,' indicating the type of numbers being discussed.
Сложите три трехзначных числа.
Add three three-digit numbers.
In this case, 'трехзначных' describes the specific attribute of the palindromes in terms of being three-digit numbers.