Here's why your question is a bit tricky:
* "Slither Sky Dragon" is not a particularly rare card. It has been printed in multiple sets, including common versions.
* Rarity is not tied to specific card combinations. There's no inherent "three rare cards" rule or theme associated with "Slither Sky Dragon."
However, you might be thinking of:
* Specific cards that are commonly used with "Slither Sky Dragon" in decks. Some of these cards could be rare depending on the set they were printed in. For example, "Sky Scourge, Assault Mode" is a powerful card that works well with "Slither Sky Dragon," but its rarity varies.
* Cards that have similar themes or mechanics to "Slither Sky Dragon," such as other "Sky" monsters or cards that involve summoning monsters from the Extra Deck. Again, the rarity of these cards would vary.
To get a more specific answer, you'll need to tell me:
* What specific card sets are you interested in? Rarity is determined by the set and printing.
* What type of "rare" are you looking for? Yu-Gi-Oh! has different rarity levels like "Rare," "Super Rare," "Ultra Rare," etc.
Once I have this information, I can help you find the specific cards you're looking for.