This verse has several uncommon words, including "marketplace" and "idle/doing nothing." The Greek word for "doing nothing" is the punchline. coming last as a sound-alike anagram of the Greek word for "marketplace," which is the previous word.
The third hour means three "hours" after sunrise in the system of antiquity, but these "hours" were not all of equal length throughout the year. Instead, the hours of daylight were divided into twelve equal parts every day, a longer period in the summer and shorter in winter. The assumption here is that there are three such periods after the first workers went out to work at sunrise.
Note that the verse doesn't mention any need on the part of the owner of the vineyard. The cause mentioned is that the people were standing idle, doing nothing. There is a strong sense here that everyone should have work to do, that they should be producing something of value with the time.
Christ's Words in Matthew as a Guide to 40 Days of Prayer.