Walden, 27, spent most of 2015 in the Independent Atlantic League after being released by the Reds' organization early in the year. He pitched well in 48 games, posting a 2.95 ERA.
In 36 games at Double-A Birmingham in the White Sox organization, Fernandez, 25, posted a 4.26 ERA with 45 strikeouts in 61 innings.
Martinez, 28, compiled a 3.21 ERA with 49 strikeouts in 67 1/3 innings last season between the Rangers' Double-A Frisco and Triple-A Round Rock affiliates.
Jamal Collier is a reporter for MLB.com. Follow him on Twitter at @jamalcollier. This story was not subject to the approval of Major League Baseball or its clubs.