1. I have three colored pencils.
2. I have three color pencils.
(Which one is right?)
3. Leather is (a) treated animal skin which is used for making shoes, clothes, bags, and furniture.
(Do we have to use'a' or not in this entence?)
1 is correct; 2 is incorrect.
3 is fine either way, but spoken in general, you can omit the word "a."