what's the condition to get the 3rd hot spring cg? i've beaten the postgame dungeon and got the first scene there
I'm not sure, but let me tell you what I found.
The text for the onsen scene is in file Map046.rvdata2. At the bottom of the file it has, additionally: "For some reason, if you defeat a fishy boss while suffering a terrible condition and enter the onsen you will get a small conversation. It's a little reward for doing things the hard way."
To test this out, I did multiple boss runs of every single boss in the game (on normal difficulty). The mysterious tentacle conflicts with every other status, by the way. I did at least two notable runs: one of which was with the mysterious tentacle, and one for every single status because I found the room in the recollection room that gives you every status after several attempts...
I wasn't able to get it. Additionally, I couldn't find any altered text for the scene in the Map046 file.
I'm not saying it's not there, but I have played games before where they included cut cg files.
Hope this helps.