I do know my Camelbak resevoir 70oz, (measures 18inch by 6 inch empty) from a Camelbak Classic, fits my Rallye Pro 2 perfect, dont know if you can still get the same resevoir as its a few years old now and no idea about the Pro 3 but I wouldnt have thought it would be much different but I may be corrected. Hope this is of some help Found this http://www.merlincycles.co.uk/Bike+...+Spares/Camelbak+Omega+Plus+Reservior_353.htm