The suspensions are for the Western Conference semifinals starting with Game 5 on Wednesday.
The penalties followed a flagrant foul call on Horry, who delivered a forearm to Suns guard Steve Nash with 18.2 seconds left in Monday's 104-98 Phoenix victory at San Antonio, evening the best-of-seven series.
Horry was suspended for flagrantly fouling Nash and hitting Raja Bell above the shoulders with a forearm. Stoudemire and Diaw were suspended for leaving "the immediate vicinity of their bench" during a melee.
Stoudemire is averaging a team-best 23.9 points and 12.1 rebounds per game in the playoffs. Diaw is averaging 7.2 points. Horry, playing in his 15th straight playoffs, is averaging 6.4 points this postseason.