Well, I myself change my oil rather often... every 20.000 Km, knowing that there is still plenty of reserve left. I could go even up to 30.000 km without hurting my engine. By the way, the oil is nothing special and not very expensive. In fact it is the right pick from the hardware store around the corner.
You think you can do the same with your engine and the same oil? Maybe yes, maybe not. For sure, 3000 miles is a very short drain interval. But to extend oil drain to 20.000 km or more, many things must fit perfectly together, otherwise you'll see strange effects like Toyota sludge.
I cannot recommend you to boost up your TBN with extra additives. It would take me a lot of time to explane why I would not recommend this, just believe me that there is reason behind that recommendation.