2pins should be connected to the coil. The other 3 are from the contacts (probabely a SPDT contact)
When 12V is applied to the coil, the contact will be swapped.
There should be a drawing or something on the relay that shows wich pins are for the coil and wich for the contact. If there isn't, you can measure it. The coild should measure some resistance (few 100's of ohms), but not infinte or 0. If you find these 2 pins you got the coil, the other are the contacts.