The Marlins are such big underdogs in this spot largely due to reputation and the Nationals pushing for a playoff spot. 
But there are a lot of circumstances here that point to an outstanding value play backing the home underdog. 
Washington has won the first two games of this series. This is the Nationals' final regular season road game. They return home to host the Phillies on Monday, closing out the season with eight home games. So the Nationals might not have their full concentration while also taking the Marlins too lightly in this Sunday get away day spot. 
The Nationals are pitching their No. 5 rotation guy, Austin Voth. He has a 3.77 ERA on the road this season. Washington is 1-5 in Voth's last six starts. 
Pablo Lopez starts for Miami. He's been a much better pitcher at home where his ERA is 3.22. Lopez also has a 2.76 ERA in day games. He's coming off a Monday performance where he held the Diamondbacks to three runs on five hits in six innings. 
The Marlins do not have a good bullpen. However, the Nationals have the highest bullpen ERA in the majors at 5.79.