18500s won't fit in a C3, you'll need to use a pair of 17500s, go with the AW brand protected 17500 cell, they perform right up there around their 1100mAH capacity rating and will come pretty close to the runtime of 3xCR123s.
SureFire bodies are bored to fit CR123s reasonably snug to reduce rattle, that means ~17mm, so an 18mm cell won't fit unless you bore the stock body, or replace the body with a leefbody.
Oh- and please make sure to use a good charger, or at least read up on the limitations and safety issues with cheaper chargers. I highly recommend the Pila IBC charger, it's a little pricier, but you can sleep at night knowing your cells are going to be charged in a safe manner. Li-ion is very safe when charged and discharged correctly, unfortunately, not all loose cell consumer chargers follow the rules. A mistreated li-ion cell can become a violent little fire breathing toxic gas emitting stick of doom.
Eric