Thanks for your response.
Yes, by default, first expression is taken in X-axis, second expression in Y-axis and third expression as size of the bubble. I'm able to define and get the desired result in the basic expression tab view but when I shift to "Advanced mode" the option of defining an expression in X or Y axis is not available. Hence, the confusion.
I guess there is some inconsistency in Qlikview tool. Would point this out in issues forum.