In my research I've found Carrefour - they seem to offer a good deal for my and the OP's needs. Can you give any feedback on this?
Spain - carrefour - Prepaid Wireless Internet Access
Here is the plan that interests me:
"A 'bono 3GB' add on is also available:
29 Euro for 30 days, 3GB at full 3G speed, thereafter speed is reduced to 128k
Make sure that you have enough balance on your SIM card (at least 35 Euro).
Enable the plan by sending SMS to 22864 with the text ALTA BONO 3GB
Check the amount of data you have used by sending BONO 3GB to 22864 (5 free SMS per month)"