Compounds having the structure of Formula (I) or pharmaceutically acceptable isomers, racemates, hydrates, solvates or salts thereof, where A, R1, R2a, R2b, R2c and R3 are as defined herein, are useful in the modulation of IL-12, IL-23 and/or IFNα by acting on TYK2 to cause signal transduction inhibition, as well as to pharmaceutical compositions containing the same and to methods of their use and preparation.