Positive:
* Beautiful
* Comfortable
* Delightful
* Exquisite
* Fantastic
* Magnificent
* Melodious
* Mysterious
* Optimistic
* Picturesque
* Splendid
* Wonderful
Negative:
* Abominable
* Disastrous
* Horrible
* Miserable
* Ridiculous
* Unfortunate
Descriptive:
* Ancient
* Bizarre
* Cautious
* Circular
* Curious
* Dangerous
* Electric
* Exuberant
* Fragile
* Harmful
* Massive
* Noxious
* Precious
* Specific
* Terrific
* Violent
* Whimsical
Other:
* Academic
* Artistic
* Automatic
* Classical
* Dramatic
* Economic
* Historic
* Political
* Scientific
* Strategic
This is just a starting point. There are many other 3-syllable adjectives out there. You can also use an online dictionary or thesaurus to find more.