Echoing real life, it's not a party with less than a quartet. Mario Party will frequently substitute CPU players for empty controller ports. Players have to watch the CPU play just like they would another person, only you can't reach over and smack the drink out of the CPU's hand when it steals all your stars. Your smug friends, on the other hand, are just asking for it.
One solution is to just plug in extra controllers and have people play multiple characters. Everyone else is advised to keep knitting handy to pass the CPU turns. You know how much fun it is to watch other people play boring games? Imagine being forced to watch AI opponents run through their turns. Over and over.
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