Creatures on the battlefield whose base power and toughness were modified by Harmonious Archon (most notable the 1/1 human tokens it creates) remain base 3/3 after the Archon is destroyed.
Note that creatures that enter play after the Archon is destroyed (like, say, the frog token made from killing it) enter the battlefield normally.
@Oktagon_Support @Tombstone First bug of Eldraine!
//Edited Title -Brigby