our urbanization went through a similar process in 2008. However, 3 phase electricity is normally used for commercial properties, or very large properties, are you saying that this is what has been recommended for your private property?
A standard house without pool normally has sufficient with 5.75kw and the standing charges are cheaper than the next one up which is just under 10kw.
However, if you go to Iberdrola.es you will find all the relevant prices there, you may also be interested in opting for Tarifa Nocturna, if you live here, it's similar to Economy 7 back in UK, except that it runs for 14hrs a day at a cheaper rate (currently less than half price).
If in doubt why not call Iberdrola and ask them to clarify for you, they are normally very helpful.