The main difference between those three categories is the number of components that have been extracted from the plant and have been included in the final product.
In Full-Spectrum products, both CBD and various other components have been added to the product. Among them, terpenes, CBG, CBN, and even a small quantity of THC might be present.
Broad-Spectrum products are composed differently, and while they will have the various types of cannabinoids and terpenes, they will have no THC whatsoever.
If you want to have pure, or at least almost 100% CBD-based products, you should take Isolate, as these products have almost zero presence of any other component.