How can I refer to multiple equations with "Eq. (i)" style?
In my old copy of the AIP style guides (American Institute of Physics) they use (Section IV Mathematical Expressions, C. Displayed Equations, 2. Equation numbers) the format of 


  Eqs. (3)-(5)


in their text. 

But, as noted, check the author guidelines or style sheet of your particular journal (or just look in the articles for an example).