If you were to add a third pattern, such as polka dots or checkers, you should make sure that the shapes composing the patterns are of a different size than the two patterns you are already wearing.
So if you want to add a color, go with a dark, muted one like.
Denim and Khaki are a combination that you should try once in your life.
If you are someone who does not prefer many colors in their outfit, such a shirt and pant combinations for men will definitely suit you.