If I tell you my cat is “young, fluffy, loud, stubborn, fast, destructive, capricious, and orange,” I’ve overloaded my description with adjectives, and you won’t know what part of that information is essential.
The Rule of Threes happens at the line level, especially with description. If I tell you my cat is “young, fluffy, and orange,” those three images give you a solid description. If I tell you my cat is “young, fluffy, loud, stubborn, fast, destructive, capricious, and orange,” I’ve overloaded my description with adjectives, and you won’t know what part of that information is essential.