Radiator cooling problem??

Last month i bought a Murray radiator from oreillys for a 69 dart 318 and the difference between the new one is that it is a 2 core and the old one is a 3 core. The new radiator over heats fast and gets up to 210 but the old radiator never past 190.I dont know what seems to be the problem but i've invested in water wetter,antifreeze and new radiator hoses but it still overheats,, i need some information about the radiator brand or if i should get something else for it to help cool it down but everything i've done so far it dosent help much..