Verb tense: I (have been/was) away...

I have made up the sentences below.

(1a) I have been away for a few weeks. I came back this morning. (correct)

(1b) I was away for a few weeks. I came back this morning.

(2a) I have been away for a few weeks. I came back yesterday.

(2b) I was away for a few weeks. I came back yesterday. (correct)

(3a) I have been away for a few weeks. I just came back. (correct)

(3b) I was away for a few weeks. I just came back.

My non-native English speaking friends told me which ones were correct, which I marked down in brackets. They think you have to use the simple past in (2b) because if your return happened yesterday or before that, "was" would be the correct tense.

I think either tense in ALL of my sentences is okay. Could you please tell me which tense is correct in each pair of sentences? Thank you.