They are all identically constructed. The difference between them ist hat the H80 has a 120mmx120mm radiator while the H100 has a 120mmx240mm radiator and the H120 has a 120mmx360mm radiator. They take up one, two and three fan slots respectively with each step up increasing purchase cost and cooling power.
The H100 is the most popular and the one that I use in my current desktop. Previously I've used custom water solutions which while better, are a massive hassle