Is a grandson of the same name as his father and grandfather named the 1st or the 3rd and why?

I found out on my own: when a grandson is born & is given the same name as his father who is ' Jr. ' & his grandfather is ' Sr. ', the baby becomes the ' III ' , the father becomes the ' II ' & the grandfather becomes the ' I ' .

While the above may hold true is some cases, it is perfectly normal for the grandfather and father to maintain their "Sr/Jr" titles and for the son to be given the III title. My grandfather was Michael Sr. My father was Michael Jr. and my little brother was Michael III.