and whats the chance with title power 3?
In Pokémon Scarlet and Violet, Pokémon originating from the following sources can never initially have marks:
•Pokémon caught in static encounters, such as Legendary Pokémon, Gimmighoul, etc. •Pokémon caught in fixed symbol encounters, such as wild Tera Pokémon, etc. •Pokémon caught during Tera Raid Battles •Pokémon hatched from Eggs •In-game trade Pokémon •Gift Pokémon -Excluding the Shiny Munchlax rewarded from Ogre Oustin' •Event Pokémon -Excluding instances where the event data assigns a mark instead of a ribbon, such as The Hidden Treasure of Area Zero Hisuian Zoroark or Glaseado Mountain Cetitan
Source. According to Bulbapedia, the answer is no, since Ogerpon is a static encounter at the end of the DLC. This also makes a lot of sense. Most likely, the Reddit person is mistaken about the mark. The chances of catching a Pokémon with the Destiny Mark on your birthday with a title 3 sandwich is 1 in 7.2(not too far from my 15% guess[ok, pretty off]). Hope this helped!