Only one of the 28 passengers escaped unharmed.
Police have confirmed the identities of the dead women as Julia Sánchez Hernández, 58; Carlota Ortiz Girón, 38; and Ana Belén Faustino Ramos, 29.
They were travelling in the minibus from Oviedo to Madrid, having set off yesterday morning.
Of the other 24 who were injured, one had to be flown to Salamanca hospital by helicopter.
The remainder were treated in accident and emergency at León hospital, and those remaining on the ward are expected to be discharged today.
All passengers on the bus, which belongs to the company Milo, were on a circuit tour of Asturias since last weekend was a bank holiday in Madrid.
They were aged between 26 and 74.
Police say the tragedy occurred at around 13.35hrs yesterday, at the exit of the El Negrón tunnel at kilometre 103 of the AP-66.
The causes of the accident are unknown, although the road surface was said to be slippery due to recent heavy rainfall.
