A boy can only be potty trained in 3 days as long as the boy is actually ready to potty train and can get to the toilet themselves and pull their pants and underwear down.
When it comes to potty training a boy in 3 days the boy should be at least 2 years old to 3 years old before they will actually be able to potty train in only 3 days.
When you're potty training the boy put them in underwear and put some plastic pants over the underwear so that you don't get pee all over the place.
Or keep them in regular diapers and remind them often to go to the toilet.
When you notice the toddler about to poop or pee then take them to the toilet and remove the diaper and put them on the toilet and explain to them that they need to go to the toilet to pee or poop.
It can take longer than 3 days to potty train your boy no matter the age but eventually they will get it.
Pull Ups are really just less absorbent diapers so I always just used regular diapers on my kids when they were potty training.
I would watch for signs that they were peeing or pooping and then take them to the bathroom and remove the diaper.
Then when they were done I'd put the diaper back on them if it were not dirty or wet and if that was the case they'd get a new diaper.
After a month or two of the kid keeping their diaper dry or nearly dry then they would go completely to underwear.