As mentioned, the voltage increases from 7.4 volts to 11.1 volts, but also remember that this means your power system will draw more current so if it is rated for 2s you will need to use a smaller prop to use it with 3s. Obviously the pack will be heavier then a equal capacity 2s pack, but it will also have proportionally more total energy.

Ryan Lefevre
www.CommonSenseRC.com
The Go To Guys For Electric Power