My family speak 2 languages, When should my child learn 3rd language?

The best time to pick up a second (or even third or fourth) language for a child is before the ages of five to six years old. It is regarded as a "peak period" where their language acquisition skills are at their best. They can easier pick up the different phonetic pronunciations (believed to be at their sharpest before 3 years old). It is important to have someone to speak to your child in the third language so that he/she can practice and pick up the language. Also, have different people to consistently speak one language to him/her to minimize confusion. It will be good to start early so that he/she will be exposed to the different languages even if he/she has yet to start talking. Babies can learn through listening and would pick up patterns along the way. Here is a great link with some tips on how to can facilitate his learning: http://www.parents.com/toddlers-preschoolers/development/language/teaching-second-language/