Depends on location but anything from 800 Euros to thousands.
Although here is a search with Kyero and the cheapest 3 bed in Barcelona city was 1,000 euros per month on a long term let, but unfurnished and only 80sq meters, which is really small.
http://www.kyero.com/browse/barcelona-city-apartment-long-let-bed3loc4082pag1pgr1ppp20slt1srt4
I’ve known people struggle on a monthly income of 2,500 Euros a month.
Barcelona suffers like all major cities that are in demand, the general population cannot afford to live there with the wages there on.
The average wage in Spain is around 1,000 euros, higher in Barcelona, but the only way to survive is on a large income, which always leads me to ask why are they paying a foreigner big wages, especially in a country full of good Universities and with high unemployment levels in general and also in its graduate population.
Sorry to put a worrying thought, but I’ve come across many who got promised good incomes, which never arrived, it happens a lot.
My advice would be to arrive and start the job for a month, maybe more before actually moving to Spain, that way if it is a bad job, you can return relatively easy.