"I want wood and no stone or the composite stuff. However, finding a solid piece of hardwood at that length is both difficult and very expensive."
From what I gather, you want to build a solid wood top 12' long.
No problem. Just go to a few local cabinet shops and ask where they get their hardwoods. Boards 12' long in hardwood are easily had and sometimes hardwood solid tops that size also.
If you want to use 3 4' sections, why dont you frame the outside with dividers and make the 4' sections as panel inserts. More or less a frame and panel design.
If you have to ask this question, I can safely assume that you do not have the skills and probably not the tools to make a 3' x 12' flat top. So whatever you try will be a gamble. Keep that in mind.
Actually I personally would not use hardwood for a desktop. It damages easily. I would use MDF and a nice plastic laminate (formica, Wilsonart, etc.) and surround the whole thing with a 2" x 2" frame. That would be very durable, easily cleaned and the wood look from the hardwood framing edge. Just a suggestion.
Tony B Retired woodworker, among other things.
"Strive for excellence and settle for completion" Tony B