I created a superflat world with the present: 3;minecraft:bedrock,49*minecraft:sandstone;2;
(Generate structures is off, I don't know weather it shows that in the present or not.)
I'm not sure if this will be of any help, but here is the seed:
-288029455

In places around the world, there were chunks that had a bedrock layer at the bottom, two dirt blocks in the middle, and a grass block on top, which is the same layout as the default superflat world present.

I hope this helped, thank you.