I don't have any information about a Magic: The Gathering keyword or ability called "Three Autostubs." This isn't a real MTG mechanic, keyword, or ability that exists in the game.
It's possible you're thinking of a different term, perhaps a misspelling or a confusion with another keyword. Some real MTG mechanics that might be what you're after include:
- Cascade – exile cards until you hit a cheaper nonland spell, which you may cast free
- Storm – copy the spell for each spell cast before it this turn
- Annihilator – defending player sacrifices permanents when this attacks
If you let me know the correct keyword or describe what the ability does, I'd be happy to give you a clear, Commander-focused explanation of how it works and where players commonly trip up on it.