Final answer:
Pronoun agreement is the third syntactic indicator to determine whether a word is a noun or not.
Step-by-step explanation:
The third syntactic indicator to determine whether a word is a noun or not is pronoun agreement. Pronoun case refers to the function of pronouns in a sentence. Subjective case pronouns act as the subjects of a sentence, objective case pronouns function as objects, and possessive case pronouns indicate ownership.