This is a FAQ:
//someName[3]
means: all someName elements in the document, that are the third someName child of their parent — there may be many such elements.
What you want is exactly the 3rd someName element:
(//someName)[3]
Explanation: the [] has a higher precedence (priority) than //. Remember always to put expressions of the type //someName in brackets when you need to specify the Nth node of their selected node-list.