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

Children go through various stages of language development and really starts from infancy. So for instance, if your baby is around 3 months, he may respond to familiar voices and will listen closely to anything that he is not familiar with. To express delight at seeing his parents or anyone familiar, they usually smile or make a resounding noise. If your child is between the ages of 4 and 6 months, they respond to small words like 'yes' or 'no.' They may even catch up on consonants. If your child is between the ages of 7 and 12 months, they are already familiar with their name and will look at anyone who calls them. As they grow, they catch up on their mother tongue, and are able to talk fluently in it by the ages of 2.5 and 3. Now, as far as the third language is concerned, you can start tis process as soon as your child hits the '4 year' mark. This is the time when you can easily make your child learn a new language (apart from the mother tongue). Do take a look at this article for more information on how to develop language from infancy: http://www.theindusparent.com/how-to-boost-language-development-in-infancy