As per my knowledge , we can set the chart completely as stacked or independent bars but we don't have option of selective stack and bar show. You need to go with all the expression as stacked or individual bar of each one .
I had the requirement a few times to have two expressions stacked and a third one beside it - not stacked on the first two.
It is only possible in a combochart to start with.
Unfortunately it's been a while since I did this, so I don't remember all the details.
But I think that is it: You need a combochart, then you add two dimensions to it and then you have to somehow make sure - via IF_expressions or so - that two of your expressions are displayed on the same value of the secondary dimension - thus they will automatically be stacked - and the third one is displayed on the other value of the secondary dimension.