The brothers, from Pakistan, were arrested in Lleida for spreading jihadist ideology using social media, Spain's Interior Ministry said.
Aged 25, 29 and 31, the three lived in the same house where they worked to produce and distribute jihadist propaganda, including videos showing executions by Islamic State as well as supporting the Taliban and Pakistani extremist groups.
Investigations show the brothers had operated various social media accounts and had successfully recruited others to "the glorification of terrorism", said the statement.
It is the latest of a series of arrests of people in Spain accused of promoting jihadism. Since the start of the year, 29 people suspected of belonging to or acting for Islamist militant groups have been held in Spain.