A three letter word that could mean clever is sly.
Eat.
aid
ton
sot
No, because for example the cat can have cleverness but when you are intelligent you know the answer to most of life's questions.
There is no word, but SRO (standing room only) means the same thing.
The three-letter word for energy that is the same spelled backwards and forwards is "rad."
The letter 'I'
bee
same
Wan.
there isn't any