Eternal Decks · Board game rules
could you put 4 rares in a cube to do stage 2 and 3 for Stage C?
Quick answer
Yes — Rare Cards can be placed adjacent to each other in the Field and used to help complete Puzzle Cards of any level, including Level 2 and 3, since they count as any color/number.
Details
Rare Cards are special Number Cards that "can be played as any of the five colors and/or any of the nine numbers" and "can be next to each other in the Field," ignoring the normal same-color/same-number placement restrictions [p.6 — The Secret of Life Energy] [p.10 — Taking A Turn].
In Stage C specifically, when solving Puzzle Cards, "Rare Cards, the Minotaur Ability Card, and the CAMP 4th Lap Bonus Card can be used as any color in a puzzle," so you're free to use multiple Rare Cards to satisfy puzzle requirements for any of the four puzzle levels (including Level 2 and Level 3) [p.33 — Puzzle Completion Rules].
There's no stated limit on how many Rare Cards you can use together in a puzzle or row — since Rares can be adjacent to each other and each can independently be treated as any needed color/number, using 4 Rares together is legal under normal Stage C rules.
Watch out: This changes in the advanced Stage C+ variant ("COMBINATION"). There, "Rare Cards, the Minotaur Ability Card, and the CAMP 4th Lap Bonus Card cannot be used to complete any puzzle" [p.50 — Giant Puzzle], so if you're playing C+ instead of base Stage C, Rares won't help you complete Level 2/3 puzzles at all.