Also, keep in mind that I've already conceded to the most ultra-conservative estimates, as defined by you. In reality, North Korea likely has a lot more than 3 warheads
Yes, but can they miniaturize and shape these warheads to be fitted on missiles effectively?
Once North Korea develops the long-range missile re-entry capability of warhead delivery, there is no limit to how many rockets it could build. Let's take a mid-point between the estimate given above, and a minimum of zero, and suppose NK eventually builds 40 nuclear-tipped ICBMs. Now we're talking about 40 million, rather than 5 million. Either way, it's certainly comparable to a Soviet strike circa 1985.
You really think the USA or even China will allow North Korea to build up a seriously powerful ICBM force? Keep in mind these NK rockets are going to be of lower quality and probably few of them will actually be able to reach the West Coast. Also, even 40 nukes would not compare to a massive Soviet strike circa 1985.