ST. LOUIS -- The Cardinals added three players, including top prospect Oscar Taveras, to their 40-man roster on Wednesday to protect each from being eligible for the upcoming Rule 5 Draft.

The 21-year-old Taveras was an obvious add for the Cardinals, who have no more highly coveted player in their Minor League system. Outfielder Mike O'Neill and infielder Greg Garcia were also placed on the roster.

There were roster subtractions, too, as left-hander John Gast and infielder/outfielder Jermaine Curtis were removed and placed on the Triple-A Memphis roster. Infielder Ryan Jackson was claimed off waivers by the Astros. The flurry of moves leaves the Cardinals with 36 players on their 40-man roster.

Players eligible for the Rule 5 Draft -- which will be held Dec. 12 -- had to be placed on a team's 40-man roster before 11 p.m. CT on Wednesday in order to be exempt. Eligibility depends on when Minor Leaguers were drafted and the age at which they started their professional career.

Players first signed at age 18 must be added to 40-man rosters within five seasons or they become eligible to be drafted by other organizations through the Rule 5 process. Players signed at 19 years or older have to be protected within four seasons. Clubs pay $50,000 to select a player in the Major League phase of the Rule 5 Draft. If that player doesn't stay on the 25-man roster for the full season, he must be offered back to his former team for $25,000.

In other words, an international player or high school draftee signed in 2009, assuming they were 18 or under as of June 5 of that year, must be protected. A college player taken in the 2010 Draft is in the same boat.

Taveras continues to recover from August ankle surgery, but is expected to be fully healed by the time the Cardinals open Spring Training. He'll compete for a roster spot immediately and should retain top prospect status when MLB.com unveils its 2014 rankings. Taveras hit .306 in his first taste of Triple-A this season, but was limited to 46 games because of the ankle injury.

O'Neill, a 31st-round selection out of the University of Southern California in 2010, started the season in Double-A before earning a midseason promotion to Triple-A Memphis. The 25-year-old batted .320 and posted a.431 on-base percentage in 98 Double-A games. After the move up a level, O'Neill hit .295.

Garcia, who participated in big league Spring Training for the first time in 2013, spent the entire season in Triple-A. He started games at three different infield positions, but logged the majority of his playing time at short. In 116 games, Garcia, 24, hit .271 with 23 doubles, four triples, three homers and 35 RBIs.

Gast, Curtis and Jackson all enjoyed short stints with St. Louis in 2013. Gast joined the rotation in mid-May and made three starts before a left shoulder strain forced him to the disabled list. The lefty later underwent season-ending shoulder surgery.

Curtis made his Major League debut in April and accrued five total plate appearances during two different callups. Jackson made the Opening Day roster because of an injury to David Freese, but was sent down to Triple-A one week later. Though the Cardinals were seeking shortstop upgrades all season, Jackson did not return to St. Louis until September, a sign that the organization did not see him as a suitable fit.