A 3 speed shouldn't be bad on the highway as long as it has a locking torque converter. My Buick has a 4 speed overdrive automatic in it, but the mileage is down right now because the TC isn't locking.
Depends a whole lot on the actual gearing, Marvin for instance turns at 2600 at 60mph with the TC locked up with a 3 speed auto. Many close ratio 5 speeds have higher revs at 60.
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Usually the 3 speed autos don't rev disgustingly higher on the freeway because they put a different final drive gear in it to make up for the lack of overdrive(but they can only do so much so it still revs a bit higher). Where you lose the economy is mainly in town because you end up with 3 wide gears starting with a tall first gear. 3 speed auto cars are usually the ones that leave first gear at 50-60 floored.
The opposite of that is why I want to go with a 5-speed auto in my car. I change out the rear from 4.62 to 4.10, keep the first gear ratio from crank to wheels but when I'm in 5th gear on the freeway i turn 500-600 rpm slower.