Mazzaro was 4-3 with a 5.73 ERA for the Royals while filling in as a spot starter because of injuries last season. Robinson struck out twice in his first four major league at-bats. Both were designated for assignment on Nov. 20.
The 21-year-old Santos pitched for both Dominican Pirates clubs last season, combining for a 6-3 record and a 2.44 ERA in 14 games, including 12 starts. In 62 2-3 innings, he allowed 42 hits, walked 20 and struck out 74 while holding opponents to a .184 average.
Rico, who turns 19 on Thursday, was 0-1 with a 7.04 ERA in 11 games for the Dominican Pirates last season.
The Pirates also acquired right-hander Zach Stewart from the Boston Red Sox for a player to be named.
Wednesday's deal came eight days after the Red Sox designated Stewart for assignment. In two starts with Boston last season, he was 0-2 with a 22.24 ERA in 5 2-3 innings.
The 26-year-old Stewart had been acquired from the White Sox for infielder Kevin Youkilis on June 24. With Chicago last season, he was 1-2 with a 6.00 ERA.
Stewart made 11 starts for Boston's Triple-A team at Pawtucket, going 3-5 with a 3.94 ERA.
In 33 major league games with Toronto, Chicago and Boston, Stewart is 3-10 with a 6.82 ERA.