Harden, perhaps the most injury prone of the trio, made two starts with the A's before suffering a right shoulder strain, which sent him the disabled list on April 10.
Harden has had to answer questions about his health the past three years. He remained pain-free and on schedule during Spring Training. In his two starts, one in Japan, he looked dominating, if a bit wild. He struck out 15 in 11 innings and gave up one run on a solo homer.
Foulke, who has not been able to pitch since April 10 because of neck stiffness, looked good early in the season. He recorded a save against the Toronto Blue Jays, but hasn't been able to get back on the mound since. He has an ERA of 2.57 and had six strikeouts in seven innings.
"My goal is to be the dominating pitcher I once was," Foulke said in March before a calf injury slowed him down.
Foulke is scheduled to pitch in a rehab assignment on Monday for Triple-A Sacramento.
"He's been down for one inning and then came back," A's manager Bob Geren said on Saturday. "He's been working out with the team, and he'll get a couple more innings. He's going to be fine."
Calero opened the season on the 60-day disabled list with a torn right rotator cuff. He began a throwing program late during Spring Training and has been throwing in games during extended Spring Training.
"He'll throw back-to-back days [at extended Spring Training] for the first time Tuesday and Wednesday," Geren said. "He'll go to the Minors, yes, and he should be ready when he's eligible."
Calero is eligible to come off the DL on May 23.
Outfielder Travis Buck (shin splints) was scheduled to start a rehab assignment in Sacramento on Saturday.
Rick Eymer is a contributor to MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.