The 26-year-old Gaub, who was acquired in the Mark DeRosa trade with the Cleveland Indians in 2008, 26, went 4-4 with seven saves and a 3.42 in 50 relief appearances for Iowa this season. He struck out 75 batters in 55 1/3 innings pitched and limited opponents to a .209 batting average.
LeMahieu, 23, was recalled for the second time this season. He hit .243 in 23 games with the Cubs earlier this season. He started the season at Double-A Tennessee where he batted .358 with two home runs and 27 RBIs in 50 games. He finished the season at Triple-A Iowa, batting .286 with three home runs and 23 RBIs in 58 games.
Montanez, 29, joins the Cubs for the second time this season. He batted .239 in 25 games for the Cubs from May 24-July 2. He batted .321 with seven home runs and 69 RBIs in 92 games at Triple-A Iowa.
All three players are available to play against the Cincinnati Reds on Tuesday night.