^^it’s true that your stars will have to align to actually take advantage of the year’s placement, but it is in fact a year’s standing rather than course by course substitutions/credit. And it only works if you do the diploma, not a smattering of classes.
With that said, most students who do the IBD are intellectually curious and would need an extraordinary reason to miss a year of college classes.
OP is asking how one finishes in 3 years. This is one way. Not necessarily at every school, but it works at some.