Here's a quote from the book South American Earthers, in the Satanoperca daemon section( I presume by 3 spot eartheater, you mean S. daemon.
"S. daemon is a black water species which prefers slow-flowing, tea colored, calm waters with sandy, muddy or loamy bottoms".
Salsberg found them in "fast flowing water.
Preferred water parameters are soft water, with a pH, between 4.5-6.5
So an aquarium with a sandy bottom would be preferred as opposed to gravel.
This species sifts sand for small edible animals, not large ones, and even though the mouth seems large, it prefers small edibles.
Another quote from S American Eartheaters (I paraphrase).
"this species requires a very fine/soft substrate, they should be kept in a group, and when choosing companion fishes, avoid excessively large species, as S daemon is not very well able to defend itself and may waste away in such company".
Take these quotes for what they're worth, but it seems to me, you are again choosing a species with water requirements similar to the Luciocephalus you just lost, and trying to put them in with a large much more aggressive species, over a substrate that will also cause stress.
Thus again, setting yourself up for failure.(sorry to be so blunt)
There are Geophagines that would better suit your water, and your intended oscar tank mate (if your tank is large enough), and substrate (unless you change to fine sand).
I would suggest those from the Geophagus braziliensus group.
For either species if you choose to poo poo the books suggestions , my hope is the tank is at least 6 ft long. Below Geophagus iporaguensus one I would think is tough enough to be housed with an Oscar