The first two versions of the sentence are fine, but there is a difference between them.
In the first, without the commas, you are directly instructing somebody to both read and understand the information. In other words, just reading it isn't sufficient.
Although you are effectively communicating the same thing in the version with the commas, the use of the paired commas makes what's enclosed in them seem less important.
Stylistically, this presentation is normally used to present something optional to the sentence. In this case, because of the conjunction, the information is not taken to be optional but it still has that form about it.
When reading it with the commas, readers will pause at each comma and tend to treat the information between the commas as an aside.
In other words, from the viewpoint of the sentence itself, it would be the same as saying, "Please read this important information. Oh, and make sure you understand it."
In practical terms, nobody would misunderstand what you were trying to convey, so this is a subtle point that may have no relevance to you in this case.
But if you want to be as direct as possible, use the first sentence.
Note that the only real difference between the first and third versions of the sentence is personal preference.