And of course DHCP is disabled on Combo2 and Combo 3, right.
Based on the prior thread and the current situation w/ wire, it doesn't appear to be a WDS related issue, so that's the good news. You might want to return to that configuration once you determine the problem.
Once again, it's not obvious why this doesn’t work. If DHCP is disabled on Combo 2 and Combo 3, and each are on the same subnet as Combo 1, then Combo2 and Combo 3 are simply switches. And as we all know, switches can be chained ad infinitum. All devices on any switch should behave as if they're all using one, large switch. The only “fly in the ointment” is that these are not just switches, but dsl modem+routers, and who knows what idiosyncrasies they may exhibit when you try to repurpose them in this fashion.
Btw, I don’t recall if you tried this previously, but does a wired client on Combo 3 get internet access? IOW, is the problem strictly confined to wireless users on Combo 3?
So something doesn't make sense here. It's some subtle issue/problem that's hard to detect without being able to examine the devices in detail. What I would suggest is doing a factory reset on Combo 2 and Combo3 and reapply your changes, so we know w/ absolutely certainty they are configured identically.
Also, what happens if you switch Combo 2 and Combo 3? I know you said Combo 3 now gets Internet access, but if Combo 2 is now behind Combo 3, does Combo 2 now have the problem? IOW, is it a problem of position (i.e., being third), or is it the specific device?