There are a number of famous dragons in German-language literature / mythology, but these usually have 1 or 2 heads. In Japanese monster films, however, there is a three-headed dragon named King Ghidorah, who is at least a little known.
The first-person narrator seems to want to mock the dragon. Correspondingly, I would choose another multiheaded mythical creature. The hydra. For ridicule we combine the hydra with "Verschnitt" or "Imitat".
Ein Verschnitt bezeichnet eine schlechte Ausgabe von einer Sache.
Ein Trottel bist du, du Hydra-Verschnitt, ...
Ein Trottel bist du, du Hydra-Imitat, ...
Even if this is not a very witty insult ^^.
Maybe a better approach
Du bist ein[bist ein] trotteliges/dummes/dämliches Hydra-Imitat