Chapter 3 opens with a verse that has an interesting phrase: "there was no open vision." It seems that unlike in various previous days, God was not speaking to the people of Israel directly through any prophet. In the previous chapter, one might wonder how the prophecy was delivered to Eli, and I suspect that in fact, that prophecy was not quite delivered. Despite the fact that in verse 13, God says "...I have told him...", it may be that Eli simply was not really listening, and the actual delivery was the through Samuel in a fuller telling in the story given here.
There's a fascinating juxtaposition here of the nature of Eli's ministry to Samuel's ministry. Eli, we are told, loses his sight. At about the same time, Samuel begins to have a new sort of "vision" that Eli never had, although in this case, it appears to be the hearing of a voice rather than something seen. God talks to Samuel, and at first, Samuel doesn't know what's going on, thinking that Eli has called him from the next room. Interestingly, despite the fact that Samuel has spent most of his life serving at the Tabernacle, he "did not yet know the LORD, neither was the word of the LORD yet revealed unto him." Is it possible that Samuel had never been taught Torah, or about God? The way things were messed up in that time, it seems a distinct possibility, but it's also possible that people reading this would have already known of Samuel, and what a great prophet he was. They would need to have it pointed out by the author that he wasn't simply born a prophet, but had to grow into it. That may be the meaning of this verse.
Once Samuel figures out--with the help of Eli, who apparently does not hear the voice of God--what's going on, God explains that judgment is coming to Eli's family. I've addressed in part before how it is that some people are punished for the sins of others, but why should Eli be punished for the sins of his sons? It seems that Eli is really in the position of high priest, and as such, he should be holding himself and his sons to the highest example. As for judging "his house forever", I'm not sure what this should mean. Other than the suffering of these three, the only mention of this judgment beyond the next few chapters is in 1Kings 2, where a descendant of Eli's loses his position as priest, but that was also in no small part due to his role in certain matters of his own choosing.
The next morning, Eli encourages Samuel to tell him everything he heard from God. When he hears it, he seems satisfied, which might be a bit of a surprise, but in some ways makes sense. After all, what is Eli to do against the will of God? Eli knows that all the accusations God has leveled against him and his family are true, and that they are serious. Perhaps it would have done some good to pray about it, but at the point things had come to, it's not likely it would have been much help.
Meanwhile, Samuel starts to become a famous prophet, and everyone knows that Samuel is speaking to the Lord in a special way that they haven't seen in a long time. This sets the stage for Samuel to be an authority on God's will, but also a person with a possible pride issue.