Question

select the three common nouns.
within three hours, nine inches of rain had fallen in mercer county.

Explanation:

A common noun is a general name for a person, place, thing, or idea, not a specific one (proper nouns are specific and capitalized). In the sentence "Within three hours, nine inches of rain had fallen in Mercer County", "hours" (general unit of time), "inches" (general unit of measurement), and "rain" (general type of precipitation) are common nouns. "Mercer County" is a proper noun (specific place, capitalized).

Answer:

hours, inches, rain