HOUSTON -- The evaluation of the Giants' prospects accelerated Wednesday as three Minor Leaguers were promoted and three Major Leaguers were subtracted.

Up are catcher-first baseman Pablo Sandoval, first baseman Travis Ishikawa and third baseman Ryan Rohlinger. The contracts of Sandoval and Rohlinger were purchased from Double-A Connecticut while Ishikawa was recalled from Triple-A Fresno.

Down are first baseman-outfielder John Bowker, catcher Steve Holm and infielder Jose Castillo. Bowker and Holm were optioned to Fresno, and Castillo, the Giants' primary third baseman for most of the season, was designated for assignment.

Sandoval, Ishikawa and Rohlinger all ascended one classification this season, increasing their stock within the organization. With 43 games left in the Major League season, this was as good a time as any for Giants management to gauge their skills. Several players stand between being potential contributors for the future or, quite bluntly, rejects. Sandoval, Ishikawa and Rohlinger have earned turns to prove themselves.

"At this point, we're going to give those kids every opportunity we can until we gain a better idea where we are," manager Bruce Bochy said before the Giants' 6-2 loss to the Houston Astros.

Sandoval, 22, has excelled enough for two seasons this summer. He hit a combined .350 with 20 home runs and 96 RBIs in 112 games for Class A San Jose and Connecticut. The switch-hitter also was selected for the World Team at the XM All-Star Futures Game.

"I want to keep my tempo going," said Sandoval, who's 5-foot-11 and 246 pounds and feels more comfortable catching than playing first base. Bochy, however, said that the Venezuela native will spend some time at first when he's not backing up Bengie Molina.

Sandoval's hitting isn't the only intriguing aspect of his profile. He's a natural left-hander, but he began throwing right-handed at age 10 because the positions he liked to play most, catcher and third base, were forbidden to lefties.

Ishikawa's progress appeared to have stalled after his 12-game stint in 2006 with the Giants, for whom he batted .292. He sank back to San Jose during the 2007 season. But last offseason, religious inspiration helped change his approach toward baseball.

"I really got stronger in my faith and I think putting all my trust in Him made the difference between this year and years past," said Ishikawa, 24.

Ishikawa hit .299 with 24 homers and 94 RBIs in 112 games with Connecticut and Fresno, including .463 with seven homers in 11 games this month at Triple-A.

"I couldn't tell you what I was doing," Ishikawa said of his offensive surge.

Rohlinger, 24, received an instant opportunity, starting Wednesday at third base. He committed two errors -- a wild first-inning throw and a seventh-inning fielding misplay -- but also stroked a seventh-inning RBI double.

"The more games you play, the more comfortable you're going to get," said Rohlinger, who hit .289 with 13 homers and 65 RBIs in 117 games at San Jose and Connecticut.

Rohlinger became the 13th Giants rookie to make his Major League debut this season, equaling the 1996 team's record for the most in a year since the franchise moved to San Francisco in 1958.

Bowker, another member of this year's rookie class, homered in each of his first two games and drove in seven runs, but had fallen into a deepening slump. He hit .117 with only three extra-base hits in his last 24 games, dropping his average from .284 to .247. He hadn't homered since July 2, a span of 96 at-bats.

"It's pretty obvious that I haven't been performing," Bowker said. "I think it was more that I was getting myself out -- swinging at pitches I shouldn't be missing."

But, he added, "I'm not going to change anything with my swing. It's more my approach. I'm going to go back to keeping things simple."

Bowker likely will return in September, when active rosters can be expanded to 40 players. Bochy, for one, will welcome him.

"Johnny was a shot in the arm," he said, referring to Bowker's torrid start. "He should feel good about where he's at this year."

Like Bowker, Castillo had struggled lately, hitting .105 (4-for-38) in his last 10 games as his average dipped from .261 to .244. He had only four extra-base hits in his final 26 games.

The Giants have 10 days to trade Castillo, release him or send him to the Minors if he accepts his assignment.

"He's done a lot," Bochy said. "There may be some interest from a contending club in him. That's what we're hoping."

Castillo, claimed off waivers on March 22, also wants another big league opportunity.

"I'm surprised," he said. "I played well for this team."