CINCINNATI -- Three Reds pitching prospects were added to the club's 40-man roster on Thursday to protect the players from being exposed to next month's Rule 5 Draft.

Robert Stephenson, the organization's No. 2 prospect, along with fellow right-handers Sal Romano (No. 17) and Stephen Johnson, joined the big league roster for the first time.

The Rule 5 Draft allows teams to pluck unprotected players from other organizations. Eligible players include those who signed before turning 19 and have been in an organization for five years, or older signees who have been in an organization for four years.

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This year's Rule 5 Draft is scheduled for Dec. 10 in Nashville, Tenn., at the conclusion of the annual Winter Meetings.

One notable prospect who was left unprotected was outfielder Aristides Aquino. The organization's No. 15 prospect, according to MLBPipeline.com, Aquino split this season at Rookie-level Billings and Class A Dayton. The 21-year-old batted .247/.290/.399 with seven home runs in 74 games combined a season after hitting 16 homers in 71 games for Billings.

At Double-A Pensacola and Triple-A Louisville, Stephenson was 8-11 with 3.83 ERA. The 22-year-old, who was Cincinnati's first-round pick in the 2011 Draft, allowed 104 hits and walked 70 over 134 innings with 140 strikeouts and a 1.30 WHIP.

A 23rd-round selection in 2011, Romano was 6-5 with a 3.46 ERA in 19 games, including 18 starts, for Class A Advanced Daytona. He did not fare as well following a promotion to Pensacola, where he was 0-4 with a 10.96 ERA in seven starts. While he had 138 hits over his 127 innings overall, the 22-year-old Romano walked 45 and struck out 88.

Johnson, 24, came to the Reds organization in the August trade that sent outfielder Marlon Byrd to the Giants. Before the move, Johnson was 3-0 with a 3.41 ERA in 44 relief appearances for Double-A Richmond.

Following the trade, Johnson worked 8 2/3 scoreless innings over six appearances for Pensacola.

The Reds now have 38 players on their roster, leaving two open spots.