Either is correct grammatically. You'd use 'I've just been talking to him for three hours' if it happened right before you made this statement. You finish the conversation, walk into another room and say to someone else 'I've just been talking to him for three hours'. You'd say 'I was talking to him for three hours' if you're talking about something that happened longer ago, eg 'I was talking to him for three hours this morning/yesterday/last Wednesday'. In this case I would omit 'just'. If you say 'I was just talking to him for three hours' it implies the talking is the important bit, and it's being compared to something else. Eg: 'I was just talking to him for three hours. I wasn't shouting at him or dancing with him or teaching him how to bake a cake, just talking.' Hope that helps!

The latter would be a better choice due to the focus you mentioned.

In my opinion, both the questions and answers are oversimplified. The simplest options are: I talked to him for three hours. I just talked to him for three hours. (Recently finished) Your first sounds clumsy. (‘Just’ puts the event in the past, but ‘have been’ is what you would say if you were still talking) Your second is ok but shifts the time focus to the past. (What were you doing in the recent past? I was just talking to him.)

Use the first one. Present perfect is used to emphasize on the duration. E.g. I have been sleeping for 2 hours. (Just woke up) He has been studying in his room the whole day. They have been working on the issue for days. It does not matter whether the action is in progress, or has ended, as long as the reference point is the present, e.g. the effects resulted from the action. A possible continuation could be: I have just been talking to him for 3 hours. I am tired and I need to take some rest. On the other hand, if the reference point is in the past: I HAD been talking to him for hours last night. That's why he looked completely exhausted in the meeting this morning.

I think the first example also one that supports the action of talking is still occurring (isn't the "have been talking" present perfect progressive?). If the conversion is actually finished I think the first example would be "I had just been talking...."