The "Rule of 3000" is a general rule that few monster cards have an original ATK or DEF of over 3000. Those that do typically have Special Conditions for being Summoned, to limit the cards. A very small number of monsters with over 3000 ATK can be Normal Summoned without Special Conditions, and with very few exceptions (such as "Super Conductor Tyranno"), they have negative side effects that can hamper their usefulness.
Most monsters that have 3000 DEF and are in Defense Position, such as Labyrinth Wall, are usually safe from attacking monsters, unless card effects that change ATK and DEF are used. The "Rule of 2000" applies to ATK more than "Rule of 3000".
Notably, all monsters with 3000 or higher ATK are usually the trump cards of their series: Horus the Black Flame Dragon LV8 for the Horus LV monsters,Volcanic Doomfire for the Volcanics Series, Ancient Gear Golem for the Ancient Gears, Judgment Dragon for the Lightsworns, and Cloudian - Eye of the Typhoon for the Cloudians, Vampire Genesis for the Zombies and Super Conductor Tyranno for Dinosaurs (just to name a few).
Monsters with 3000 ATK or more are usually used towards the end of a duel, as a last resort (such as Gladiator Beast Heraklinos in a Gladiator Beast Deck), although sometimes the deck's focus will be on summoning that card (commonly seen with Valkyrion the Magna Warrior). Super Conductor Tyranno is sometimes included in beatdown decks, as they have very few ways of stopping extremely high ATK monsters.