See this Stack Exchange question:
In the response from AProgrammer, the complete rules are listed. In your first case, the 4th rule applies (-1 is signed integer, which can represent all the values of unsigned short, therefore the unsigned short is promoted to signed integer). In your second case, the 3rd rule applies (signed integer cannot represent all values of unsigned integer, so it is changed to unsigned integer). In your third case, the -1 is converted to unsigned short and then the first rule applies.
In general I like to avoid confusion by converting all vars to the same signed type (large enough to hold the range I expect) before doing any comparisons.