A fourth victim was a child crushed by falling bags of rice in Quito.
The 5.1 magnitude temblor caused tall buildings to sway, sending capital residents rushing into the streets.
The country's Risk Management office said firemen were working to rescue workers trapped in a mine near the Catequilla archaeological site.
The Guayasamin Tunnel, which connects Quito with the nearby Cumbaya Valley, was closed to traffic due to the dust cloud caused by landslides.
The U.S. Geological Survey said the relatively shallow quake was centered 14 miles northeast of Quito at a depth of about 5 miles.
Residents said the shaking lasted for more than 20 seconds at 2:58 p.m. local time.