Episode 3 is set 3 years after episode 2, which makes it approximately 19 yrs before ANH. If for arguments sake Tarkin is say 65 in ANH then hes going to be mid to late forties in Ep3. I was under the impression that he was going to be called "Captain Tarkin". If i recall all the casting rumours the dude thats gonig to play the young(er) tarkin is merely referred to as a Young Captain on the starwars.com cast list.

I think then its fair to assume that he will be a Captain of one of those early Star Destroyer ships we saw at the end of Ep2. Perhaps he is Captain of the ship that Obi-Wan is commanding the Clone Troopers that will eventually turn on him?

Perhaps this is how Tarkin learns of Obi-Wan, with reference to Ep4, otherwise perhaps we are meant to assume that Vader & Tarkin had a nice chat sometime between eps 3 & 4, when they were bored on the Death Star, perhaps dunking biscuits & listening to Mozart?

Not a lot else to do on a space station on a Sunday afternoon except reminisce on days gone is there?