Tenant is threatening to sue me over my deposit for withholding.

[Clifton Ho]

Hi - I have a bit of a complicated tenant situation. I rented  my house out to four roommates who all signed one lease. Three of them moved out this past Aug 30. However, one roommate decided to stay an extra month and moved out Sept 30. The other three roommates are demanding that I should have returned their deposit by Sept 21. according to the laws of returning it within 21 days (property is in California). However, I read that the 21 day count down to return the deposit starts when the last person moves out of the property. Does the countdown start after the last roommate moves out regardless if the others moved out a month earlier?