I'd replace all three because they should be replaced at around 80 to 100,000 miles anyway. It will save you some time and the hassle of having to go back and replace the others as they slowly die. Usually they're caked with carbon and as there useful life deteriorates they may not be as accurate as when new. Also you may get a discount when buying 3 versus one at a time B) It's like spark plugs you wouldn't just replace one if one was bad, however, they all were close to needing to be replaced, would you?