If you have no vacant monster zones, you cannot Special Summon with “Enchanting Fitting Room”. So the Level 3 or lower Normal Monsters that would be Special Summoned are sent to the Graveyard instead. All other cards are still returned to the Deck and it is shuffled.
If you have some vacant Monster Card Zones, but not enough, select the monsters you wish to Summon with “Enchanting Fitting Room” and Special Summon them.