We are bilingual, but want to be trilingual. How do we add another language?
Assuming that moving abroad is out of question right now – you can try to enlist outside “help” with the third language, like acquiring a nanny, or sending your child to playgroups, kindergartens and schools that represents the third language. It is easier to use OPOL (One-Parent-One-Language, where each parent speaks their own language) and leaving the third to the environment.
You can, otherwise, have one partner switch between two languages if you designate certain situations or times when you speak each language. It is a little tricky but not impossible. You can build in language-islands; designate certain days in which you speak the third language; make the third language a special language when you and your child are alone (e.g. bedtime language); speak a certain language whenever a certain topic crops up (discuss school and homework); have a ‘vacation’ language which you speak only when you are on vacation, or enlist the help of extended family members (see Sarah Dodson-Knight’s column in Multilingual Living Magazine. She speaks non-native French with her nephew, thereby helping him to grow up bilingual).