A jury ruled that one of the missing signs caused the death of three teenage boys who drove into the path of an eight-ton lorry.
In an emotional sentencing hearing, at which the defendants wept aloud, Judge Bob Mitchum first announced they would receive 30 years but then added: "I believe that some mercy should be shown to you, so I'm going to give you a downward departure and suspend the sentence after 15 years.
"I believe 30 years would be to waste your life but there must be punishment to fit the crime."
Under Florida's law, the three, who could have received life, will have to serve 85 percent of their terms, or 13 years.
Christopher Cole and Thomas Miller, now 20, and Nissa Baillie, now 21, admitted during their trial that they had stolen 19 stop signs along rural roads in Hillsborough Country, east of Tampa, on 7 February 1996. But they denied uprooting the sign that led to the deaths of the three teenagers returning from a pinball bowling session that night.
The defendants said they dumped the 19 signs into a river in panic after hearing of the crash. But the missing stop sign judged to have caused the fatal crash was found lying near that intersection.
Emotions ran high during the trial when the mother of one of the three boys who was killed in the crash, Ann Hertle, said she was convinced the defendants had not uprooted the stop sign in question.
"There were just too many inconsistencies. I know my own son's reactions and when I looked into the defendants' eyes, I knew they hadn't done it," she said.
The judge said he did "not believe for a minute" the three had pulled up the signs in a deliberate attempt to kill. "But [your actions] caused ramifications that none of you may ever have expected," he said.
The three were released on bail pending appeal.