At least one of those three pitchers, all of whom are missing from the 25-man active roster, will likely be with the Athletics in Baltimore in Saturday's doubleheader.
The chances that Gallagher will be part of the rotation are slim. He threw a simulated game and will make a rehab appearance for the Sacramento RiverCats on Thursday. He'll likely make his return to the rotation the following week.
"Everything was working," Gallagher said. "I felt great. I'm getting back on track."
Gallagher, on the disabled list with shoulder fatigue, threw 35 pitches to batters Cliff Pennington and Rajai Davis.
"It's more how he feels, but he looked good," A's manager Bob Geren said. "He had good velocity and a good breaking ball."
Duchscherer (strained hip) may be ready in time for the doubleheader. He's expected to throw a side session on Thursday.
Gallagher started his day throwing long toss on flat ground in left field before taking the mound in the bullpen for his warmups. His workout lasted about 90 minutes.
Meyer was sent to Midland with an opportunity to rejoin the A's on Tuesday. The Rockhounds are tied with Frisco for first place in the South Division of the Texas League with two games left.
If Midland wins the division, Meyer will have to stay with the team for at least 10 days. He would be eligible to return only if the Rockhounds' season ended.
In related news, A's designated hitter Frank Thomas will undergo further tests on Sunday to determine extent of the damage to his strained right quadriceps. He's likely done for the season, and was seen clearing out his locker.
Geren said Thomas, who became Oakland's record 24th player sent to the disabled list, will remain behind when the team leaves for its three-city, nine-day road trip on Monday.
Rick Eymer is a contributor to MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.