I gave my yuela undress 3.
and at least while fighting with low health i see the message that undress 3 is beeing used but i think its not working...namely because she doesnt undress herself. i mean the picture of her doesnt change to nude, she just doesnt want to put off her omni armor .
so is this skill bugged on the heroines? does it work at all on them?

Works ok for me.
Here's a picture of the various stages.
Had to censor it or it will get removed by imageshack. (hopefully this one's ok)
It should trigger at 50% and 25% health.

hmm for me it doesnt, it seems to trigger (hopefully, but i'm really not sure if it even is working) undress but she doesnt actually undress:

Check your config options and make sure you have it all set to ON perhaps you have something set to OFF which may be what triggers it ?

Only 1 armor for each heroine has them visually undressing.
The skill still work in the other outfits, but they won't take their cloths off.

I think someone was going to make a patch for that, but don't think they ever did.

Each Heroine has 1 Unique Armor that has visually undress that triggers at the Percentage of Their Hit Point.
if Hp is down to approximately 50% I guess the armor is torn off a little
If Hp is down to approximately 25% The Armor is torn off completely
The Armor can be accessed in 2nd Playthrough (New Game+)
You cannot get the armor during your First Playthrough.