Half-True: A child who is younger than 8-10 years old and moves to a country where a different language is spoken can become proficient in the foreign language within 6 months to a year if they are exposed to the language on a daily basis, for example at nursery or school. But even 3 months is a bit too optimistic.
Not true: For anyone older than 12-14 years, it is much more difficult to learn a foreign language, and in will take longer than for a 8-10 year old, even if they move to another country and are exposed to the foreign language on a daily basis. (Critical Period Hypothesis)
Absolutely impossible: For anyone studying a foreign language as a teenager, for example, at secondary school or as an adult at a language school, with several hours of instruction per week, will (almost) never become proficient in the foreign language and definitely not in 3 months. The process of becoming proficient will take several years and this is the scenario that is probably the most common way how foreign languages are learned nowadays. There is a variety of other factors which also influence the progress in the language learning process, e.g. motivation, confidence, perseverance, resilience, interest.