The Nationals' first wave of September call-ups includes one right-handed pitcher, one left-handed pitcher and one catcher.
This will be Davis' second stint with the Nationals this season. He posted a 6.23 ERA in five June appearances at the big league level.
The righty was acquired by the Nationals in March of 2011 in exchange for infielder Alberto Gonzalez. Over 45 appearances with Syracuse, Davis finished 3-7 with a 3.10 ERA and 9.3 strikeouts per nine innings.
Krol also will be making his second stint with the Nats this season after putting up a 3.24 ERA in 27 appearances in his first major league action. The left-hander, who was acquired by the Nats this offseason as the player to be named in the Michael Morse trade, was optioned to Syracuse on August 21.
In five appearances with Syracuse, Krol allowed two earned runs with seven strikeouts and one walk over 3 2/3 innings.
Leon appeared in 12 games with the Nats last season in his first taste of the big leagues, and hit .267 (8-for-30) with two doubles, two RBI, two runs scored and four walks.
This season at Harrisburg, Leon batted .177 in 95 games.
As of Sept. 1, teams can expand their active rosters from 25 to as many as 40.
Manager Davey Johnson said earlier this week that the Nats planned to promote three pitchers and four or five position players. The addition call-ups will likely report tomorrow.