I don't reckion you'll need a tripple core, we didn't when we did our 18rg conversion and our's runs colder that it did with the 18rc.
It'll usually cost you around the $196 mark for a new one if you take in you're old one for an exchange.
I reckon you'd be better off going for a new one, people usually don't get rid of perfectly good radiators second hand.
If you get a new one, get them to change the angle of the in pipe so that it points slightly toward the drivers side of the car, and the same with the over flow pipe. Usually when you you order a RA28 rediator, the overflow tube has to do a u turn and the top hose points for wrong way for an 18RG thermostat housing.