Land near Ashland’s MBTA commuter rail station would be one of the best spots to build housing, shops and offices in the I-495 corridor that runs through Boston’s western suburbs, according to a state report seeking to steer growth in the region. Three empty parcels totaling 190 acres that run along the commuter rail tracks in Ashland were among a list of 42 areas highlighted in the 495/MetroWest Development Compact Plan, a study based on more than a year of discussions between state and local officials on where they want to encourage development and preserve open space and historic sites in the future.