The first one (0x6C) shows this:
This level uses no textures, consists entirely of three models, and I think it might match the unused SET file and paths that were discussed in the SADX Preview topic a while back...
The second one (0x120)... shows nothing in SALVL, and consists of four models.
The third one (0x2F8) crashes SALVL, and consists of twelve models.
It doesn't make me recall anything in the game...
A hub prototype? Or a tutorial level? The other post where it speaks of textures for tutorial levels can be an indication of the latter (it even has a loop for / due to Sonic).
My first thought was Windy Valley, because of the path with the mountain on the left side, the sunk basin and the lacing paths, but then I actually paid attention what this is STG00 not STG02. It has weird path options. Again, hub (could be the mystic ruins, that too has a sunk basin in there) or a tutorial level.
That is bizarre and doesn't fit with the colour scheme of what's in the cave. I guess that's the point.
Yes, yes, I'm just saying what I see in this, don't be mean.