tenses for the past

Hi,

I learned that when a past perfect is used in sentences with the words 'before' and 'after', the use of a past perfect is not necessary, as in:

1. I washed/had washed dishes before Mary came into the kitchen.  

Here, am I saying the two events, my washing dishes and Mary's walking in, took place at the same time eventhough my washing should have started before her walking into the kitchen?

2. I was washing dishes when Mary walking into the kitchen.

How about here?

3. I had been washing dishes when Mary walked into the kitchen.

I think, this one leaves a possibility that my washing could have stopped at the moment Mary walked in although my washing should have been started and continued until the moment of her walking in.

So between no. 2. and 3, the only difference is no. 2 leaves a possibility that my action could have been stopped, whereas, no. 1. leaves no such possiblity?      

Thank you. Can I be able to use the third sentence if I was trying to make a note of the fact that the task of washing dishes was an on-going thing, emphasizing the continuous aspect of it eventhough it was an event that probably was finished when Mary walked in?

3. I had been washing dishes when Mary walked into the kitchen.--  The washing was finished before she entered-- so there is no call for the progressive aspect.  It sounds a bit odd.

Q: I am mad at you. What had you been doing when the time Mary walked in? Were you not doing anything as usual?

A: No, no, no. I had been studying for my tests when she walked in. Really, I was studying.

Is this dialogue correct and make sense?