Why different responses to target and distractor in three-stimulus oddball procedure?
I assume you are talking about a difference in neural activity?

The difference in processing of targets and distracters does not come from their relative frequency, but from their task-relevance: you are paying attention to the targets, thus increasing gain. In auditory MMN, this attention component becomes evident slightly later than the prediction component, and might be missing completely for ignored stimuli (e.g. when the participants are watching a movie while listening to a stream of sounds). Naatanen has more than one review paper that goes into this, this one probably the most recent.