are you asking where to buy the relay from? or a cct to drive said relay from the bits you have?
Farnell will likely sell such a relay, and to drive it you`ll need a simple NPN transistor like a BC108 or 2N2222A2N2222A, and resistor for the base so the 3V doesn`t overdrive it, and a bias resistor to pull it low.
probably a good idea to have a reverse biased diode across the relay coil too.
that`s about it really 
edited to add: and don`t forget to make both grounds common for both power supplies.
are you asking where to buy the relay from? or a cct to drive said relay from the bits you have?
Farnell will likely sell such a relay, and to drive it you`ll need a simple NPN transistor like a BC108 or 2N2222A2N2222A, and resistor for the base so the 3V doesn`t overdrive it, and a bias resistor to pull it low.
probably a good idea to have a reverse biased diode across the relay coil too.
that`s about it really
edited to add: and don`t forget to make both grounds common for both power supplies.
