What is the average population growth rate of countries where more than 3 languages are used?
SELECT SUM(T3.Population_Growth) / COUNT(T3.Country) FROM country AS T1 INNER JOIN language AS T2 ON T1.Code = T2.Country INNER JOIN population AS T3 ON T3.Country = T2.Country WHERE T2.Country IN ( SELECT Country FROM language GROUP BY Country HAVING COUNT(Country) > 3 ) GROUP BY T3.Country