If it is directly describing a noun, an adjective doesn't take the plural form:
- A magas, széles, szép fák. - The tall, wide, beautiful trees.
If, however, it's being described as a quality (for example, if in English it is after the word "are"), then it takes the plural form:
- Ezek a fák magasak, szélesek és szépek. - These trees are tall, wide, and beautiful.