Usually on case run, it will launch the 1st stage .
If from the menu, if you would like to launch the 2nd stage or 3rd and so on directly , probably you need to customize pyStartCase and make a call to pxChangeStage to 2nd or 3rd as required .
First, I am curious to know the use case for this problem.
Coming to the solution : One way could is to put When conditions to skip Stages conditionally. In this case you don't have to change any flows. When rules should rely on some property set by each button to launch case from different stages.
Let me know in case you need more details on this.