You don't say how old your little guy is. But if he's under 26-30 months, he may simply not have a good enough sense of what physical signals result in what 'outcome,' or a good understanding yet of what his other choices may be.
For the first couple of years, nature takes its course and results are automatic, based on the natural peristaltic action of the lower bowel. It's awfully easy to disturb that automatic action by introducing new patterns and even new emotional tensions, just as it's remarkably easy to disturb the natural breathing pattern of the diaphragm when you are emotionally distracted (or deliberately holding your breath).
Unfortunately, when kids are maneuvered into new patterns or behaviors before they are mature enough to experience them as new choices, they can get thrown off. With bowel 'training,' this can result in constipation, difficult and painful pooping, and worse. Which, of course, throw off progress.
I applaud your 3-day approach, as long as it turns out to be a good match for your son's needs and state of readiness. It is an approach that seems to work for many parents. But you might wish to check your son's readiness against the useful checklists here: http://www.parentingscience.com/toilet-training-readiness...
There are also many comprehensive and useful training tips at that link. Good luck!