A three wire relay has to have a hot wire (12v), and ground and the feed to the horns. So, if you put 12v on the center and right terminal is hot too, then one of those two is ground and the other is hot. The left terminal is the feed to the horns.
Remember, to have a complete circuit, you have to have a hot and a ground. Try this. Put 12v on the center terminal and ground the right one. Then see if the left one has 12v on it. If you get negative 12v on the left terminal, reverse the center and right leads.
Are you sure there's no markings on the relay that might give you a hint as to what goes where?
Unfortunately, my shop manual only has the standard wiring diagram for a single horn. That feed from the ammeter through the horn to the horn button, which went to ground.