I don't think you can get a 3 pin Relay, unless the case is metal and earthed. You should go for a 4 pin SPNO (Single Pole Normally Open) Relay. I just had a look and the terminals on the relayand they should be numbered 30, 85, 86 and 87.

Relay Switch

30- Supply for the Fan (via fuse from Battery)
87- Supply from the Relay to the Fan

Relay Coil

85- Supply Signal from the Dash switch
86- Earth.