The number of Greater Boston homeowners who are “underwater” on mortgages — meaning they owe more than their properties are worth — keeps falling. Zillow, the online real estate data company, reported Thursday that the percentage of homeowners with negative equity fell to 12 percent in the third quarter — nearly 45,000 homes — compared with 17.5 percent during the same period last year. The chart shows the downward trend in selected municipalities.
