Hi,
Rixxy, I think things have changed a lot since your children were in school here. My son, who was born here, is 4 and is in state school in Andalucia and started aged 3 last September. As Maria pointed out, this is classed as the second cycle of nursery or infant education - the first cycle is up till 3 years old and takes place in nursery (known as guardarias). There are state and private nurseries but state are more difficult to get into, depending on the area.
Although he is now in the second cycle of nursery education this takes place within the primary school, although in his school they have a separate entrance for those under 6. I believe this year the school year will start earlier, September 7. I understand that is not obligatory to send your child to school at the age of 3 and that the state is not legally obliged to educate a child before the age of 6. I am sure this is the case in all regions of Spain. I think perhaps Maria meant that at the age of 3 state education takes place in a school rather than a nursery. However, in the more populated areas here, it is wise to try to get a place in school aged 3, rather than 6, as it can be the case that the school you would like your child to go to is over subscribed and there may not be places in that particular school when he or she is aged 6. Think this is more common on the Costa Del Sol.
In Andalucian state schools there are schemes where you can get free school books from aged 6, but before age 6, you have to pay for them and also other class materials. In our school I understand for the free books, you have to apply for a cheque from the School Secretary but the system maybe diffferent elsewhere.
Hope this helps.