Actually, I believe the term is "3 row", meaning that there are 3 rows of veins in the core. The stock one has 2 rows......

 

yes....you can count them by removing the radiator cap and counting the rows inside. you may need a flashlight.

 

In reply to:

I just installed my 3 core radiator and I was just wondering, what makes a 3 core radiator 3 core???
I mean, it looks identical to my old radiator, but I don't know that much about radiators.

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Take a closer look - the core will probably be thicker than your old radiator. Like the others said there are 3 rows of cooling fins which equals more coolant in radiator whcih equals a cooler running vehicle.

Wayne