It's funny... I enjoyed Neuromancer, although I didn't read it until about 15 years ago.
And, yeah, the memory thing hasn't aged well. Thing is, 1984 was a funny time in computing, particularly when you consider the kind of computers normal people had access to.
At that point even things like PCs and the new Mac had 128 or 256K of RAM[0], so I get that 3MB must have seemed like an ocean of memory at the time. And, realistically, more than 1MB of RAM in machines you'd typically see sat at home or on a desktop was uncommon until the beginning of the 1990s.
And, although Moore's law had been around since 1965 it's hard to know how aware people outside of specialist circles would have been of it in 1984.
I suppose Gibson must have done some pretty in depth research for Neuromancer, right? But the memory thing is sort of ancillary to the story, so how much would he really have focussed on that? Probably not much.
And then do you really want to harshly judge the book on that one slightly laughable thing, in other ways, it was incredibly forward looking and almost prophetic? Doesn't seem right.
I think the sensible position is you have to let it slide and see it as a possible alternative future that never quite came to pass in that way but that which we can see strong echoes and foreshadowings of even still.
[0] In 1984 microcomputers, as opposed to, cough, "serious" computers like the PC and Mac, with 128K of RAM were still very new, with 32 - 64K being the entry level, and if you had one with 128K you were king of the hill. 128K in 1984 seemed like a ton of memory to most of us, but it's worth bearing in mind that only a handful of years before computers like the ZX81, which had only 1K of RAM, were the common entry level, so the progression was already clear if you looked at the situation in the right way, but you had to have been paying attention for a while to have noticed. I remember the first time I used a machine with 4MB of RAM in, maybe, 1990 - an Archimedes at school - and feeling like it was just this absolutely inexhaustible ocean of memory. In 1984 3MB would have felt almost inconceivably huge unless you were in the high performance computing, or maybe the mainframe, worlds.