You have presented a bit of a paradox here. In your first post you tell about using six different straight edges to check the surface plate. Now you're talking about needing the plate to check the straight edges - presumably the same ones.
The whole point behind the 3-plate method is that you can ORIGINATE a flat surface (THREE, for those who didn't pick up on that already). By definition, that means that by the very process you will end up with a flat surface (or three... ok, I'm done now). With this flat surface you can check how flat other surfaces are by comparison. The "lap" you speak of is each plate against the other two in turn.
Refer to "The Foundations of Mechanical Accuracy", Wayne R. Moore, Page 24.
They describe the process using cast iron surface plates and scraping but the procedure is identical for granite and abrasive. When rubbed together, high points/areas on "A" will be worn down by adjacent touching spots on "B". Likewise, those spots on "A" will remove material from "B". Do it enough, with sufficient rotations, the same amount with each pair, and using finer and finer abrasives (getting close to flat just the two together with some water is plenty abrasive enough) and you WILL end up with flat surfaces. You verify them against one another.
If you're a real masochist, you can do the whole process twice thus ending up with six plates. Compare between the two batches to remove all doubt.
I'd love to make a machine that does this automatically while I sleep, and sell the extremely flat plates for a hefty profit.
..... But more to the point, is this an intellectual exercise or do you really have a need for this level of accuracy? If you do, you can satisfy that need by spending A: a little money and a lot of time (originate a flat surface), or B: a lot of money and a little time (purchase new or resurface professionally). "A" is the typical hobbyist approach, while "B" is commercially favoured.
With the latter, you still can't verify the flatness without using a comparison surface of equally unknown flatness. You're just trusting that it is as good as they say it is. You can probably trust a reputable company, but when it comes to Chinese goods...... well, we've all been there, eh?