Here's a news story that Mitsu is considering bringing the sedan to North America, though their biggest concern is whether the 1.2L engine will be enough for power-hungry Americans.
Mitsubishi Motors Corp. will show a sedan version of its Mirage hatchback compact car to visitors from North America this month in Thailand, to gauge prospects for exporting the car to America.The layout of the Thailand-made car -- both hatchback and sedan -- rules out swapping the engine for a larger one.
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While there have been a few 3-cylinder hatchbacks available in the US over the years (including the current Smart Fortwo), as far as I know there has never been a sedan sold with a 3 pot. Even the iconic Chevrolet/Geo Metro went with a 1.3L 4-cylinder for its sedan version (the hatchback variant used a 1.0L three).
We don't have any official specs on the sedan yet, but the article says - not surprisingly - that the sedan is "slightly heavier" than the hatchback.