The 3% mentioned is a retention against possible CGT, but this is ONLY
held for NON-residents
. If you are a fiscal resident in Spain, there is no retention (but obviously, you may still be liable to CGT at 18%)
Until drsue mentioned POA, I was wondering what the solicitor fee was for. There is really no need for a lawyer when selling. Just make sure you get paid! If you want a lawyer or gestor to accompany you to the notary just for peace of mind, or to translate proceedings for you, I would expect to pay a fixed fee for his time (maybe something like €100?) but a percentage of the sale price??? Why? The seller's role involves signing on the dotted line and collecting the money. You will also need to produce a letter from your community (if applicable) stating that your fees are paid up to date, and a similar certificate from the town hall for the IBI, and also show the latest utility bills as paid. Unless you are unable to come to Spain to organise these things, none of it is difficult and there's no reason to pay someone else.
Usually, all the taxes and fees for the transaction will be for the buyer's account, with the exception of Plus Valia, which is a local council tax based on the increase in the value of the land during your ownership. Unless you have owned the property for a long time, this is unlikely to be a substantial amount. If you are in Spain, you do not need to pay a gestor silly money to organise this for you. Simply go to your town hall with the documents relating to the sale within 30 days of the transaction, and find the relevant office for Plus Valia; they will tell you how much you have to pay and where to pay it. Easy.