Ultimately, it is up to you when you would like to introduce a third language. Charlotte Mason’s students learned several languages in her schools. They always started with first hearing and saying a new language. When the students were able to read and write in that language, then they started another language with hearing and saying. Being able to read and write a foreign language shows that a student has become fluent enough in that language that beginning a new one will not be confusing. So, in Charlotte Mason’s schools, the students started learning a new language when they were able to read and write the current one.