I'm not sure if there is such a specific fanfiction. Fanfiction can cover a wide variety of scenarios, but this particular one might be a bit unusual or not very common. You could try searching on major fanfiction websites to see if it exists.
You can try fanfiction websites like FanFiction.net or Archive of Our Own. Just search for 'Ash Cynthia Diantha' on their search bars and you'll likely find a bunch of relevant fanfictions.
One great fanfiction could be 'The Journey of Ash with Cynthia and Diantha'. It has an interesting plot where Ash travels with the two strong trainers, Cynthia and Diantha, and they face various challenges together. They encounter powerful Pokémon and unique terrains. Another good one is 'Ash's Encounter with Cynthia and Diantha's Worlds'. In this, Ash gets to experience the different training styles of Cynthia and Diantha, which is really fascinating as he learns new techniques and strategies from them.
One common theme is adventure. Since they are both champions in the Pokémon world, they often go on epic journeys together in these fanfics. Another theme is friendship. Their bond as fellow champions is explored, showing how they support and learn from each other.
Power and strategy are also common themes. Since Cynthia is a very strong trainer, her strategies often get highlighted. And Ash's determination to become a Pokemon Master also adds to the power - related theme. They often face powerful opponents in these fanfics, and how they use their Pokemon's powers and their own strategies to win is a big part of it. In some fanfics, there's also the theme of exploration, as they journey to new and unexplored regions in the Pokemon world.
One popular 'pokemon ash cynthia fanfic' is 'A Journey with Ash and Cynthia'. It shows an interesting adventure where they team up to face powerful Pokemon and solve mysteries in the Pokemon world.
One common theme is the underdog's victory. Ash is often seen as the determined underdog compared to Cynthia's established strength, so his win represents the triumph of hard work and perseverance.
Writing such fanfiction that involves sexual content is inappropriate and goes against ethical and moral standards in most cases. Fanfiction should generally focus on positive, creative, and non - explicit relationships and stories.
Another theme could be power - up. Since all three are strong in their own ways, fanfictions might focus on how they train together to become even more powerful. They could be exploring new regions to find rare Pokémon or special items that can enhance their Pokémon's abilities. Also, the idea of them competing in a grand tournament that's not part of the official Pokémon canon can be a theme. Here, they face off against each other and other strong trainers, and the story could revolve around their journey in this tournament and how it affects their relationships.
Romance is also a big theme. Writers explore the potential romantic relationships between Ash, Cynthia, and the other characters in the harem. They often create unique love triangles or more complex relationship dynamics.