Jus word-of-mouth but, it is rare to get anything back. The tax authorities will not accept a lower selling price than their own. If you have had a property for a long time you may owe them more than the 3%.
Most sellers (if they are returning to the UK) look on the 3% as a write-off. They close all spanish accounts and bugger off. Not that I am advocating doing this of course 😉