While it's entirely possible there could be a transmission issue at the root of the issue, I agree it seems very unlikely. My guess would also be clutch actuator, possibly shift motors, or that it just needs a retrain. If the computer at one point expected one result and the transmission was in another state, it can throw the 3 bars.
Assuming you were given an estimate, did they describe the issue there?
Edit:
I can't recall seeing any threads indicating an actual issue with the transmission itself as they are usually pretty reliable, but that doesn't mean it can't be the transmission, just seems very unlikely.
If you can get the diagnostic codes, or have someone rescan the CAN bus to get the codes, your chances of getting better help increase significantly.
To try and help you understand how the tranmission in these work let me give a brief description.....
The transmission is a manual transmission, with a clutch. In general it is the same as you would find in a car with a clutch pedal and a stick shift, except in these cars those are operated by electric motors.
Under the car, attached to the tranmission is an electric motor which operates a piston. That piston pushes the clutch in to disengage the clutch; it is the clutch actuator.
Under the airbox is another set of motors, also attached to the tranmission but on top of the tranmission, these are the shift motors. The car's computer tells them which way to turn to select which gear to shift the transmission into.
If either the clutch actuator or the shift motors aren't working right, you'll get the 3 bars. There are methods to cause the car to relearn what state the transmission is in and reset itself usually referred to as "re-teach", "re-learn", etc when there's a problem. If there's an underlying issue, the re-teach will fail until that underlying issue is resolved.
Last but not least, depending on where you are there might be a Smart specialist in your area who may be more familiar than a general mechanic and, in some instances, a better choice than even the dealership.