Making relief pitchers be forced to pitch a minimum of 3 batters would remove a lot of the strategy of the late innings, and it would leave the defense at a huge disadvantage. Say this rule was enforced, and then 3 LHH were to come up, you would think: "This is the perfect time to use the LOOGY!" However, that wouldn't work because the manager can change all 3 batters for RHH, while the opposing manager can't do anything about it.
Personally, I don't mind having multiple pitching changes in an inning, it's a part of the game.