I see you're a newcomer to these forums, so may I recommend you have a look at the Rules/FAQ for these forums and the "sticky" thread at the top of this forum, which gives rules and guidelines particular to the English only forum.
Among those rules you will find one asking you to give us your own opinion when posing questions such as this. The danger is that by giving you the answer to questions such as this without your expressing an opinion, we will be doing homework for you.
Can you tell us whether you think there is any difference? If not, what do you think they all mean?
Thank you for giving your opinion. It is helpful to us because we can directly address the problem you are having.
Your understanding of the second and third of these sentences is perfect, however the first one, "back in two hours", means that the speaker is saying that they will be back at six o'clock. They maybe back slightly earlier or slightly later, but it will be quite close to six o'clock.