It is a cup for low level Pokémon between level 15 to level 20. The sum of the levels of the three Pokémon chosen cannot be more than 50. Technically, all Pokémon except for Mewtwo* and Mew are eligible to enter. However, getting strong Pokémon at that low of a level can get quite complicated. While some are catchable at low levels in certain versions (e.g. level 15 Dragonair in Pokémon Yellow), others are more complex and involve trading with in-game Trainers, then trading these Pokémon to different in-game Trainers in different games. Coincidentally, many of these involve Pokémon Yellow.
Ignoring glitches and tradebacks from Generation II, the following Pokémon may be legitimately obtained and entered into the cup.
A Pokémon with a * next to the version can only be obtained by trading across different versions, while a Pokémon with ** can only be obtained in the Japanese versions with the Japanese Blue.
In the Japanese Blue, trade a Kadabra for a Graveler at the Cinnabar Island lab. An eligible Golem cannot be obtained in the international Generation I games.
Catch a low level Golduck, then trade it for Rhydon at the Cinnabar Island labY. Trade the Rhydon for a Kangaskhan at the Cinnabar Island lab in the Japanese Blue, then trade the Kangaskhan for a Muk in the Cinnabar Island labY.
Catch a low level Golduck, then trade it for a Rhydon at the Cinnabar Island labY. In the Japanese Blue, trade the Rhydon for a Kangaskhan at the Cinnabar Island lab.