I have hosted several exchange students (high school) from Europe and nearly all of them come over speaking three languages. We had a girl from Germany who spoke German, French and English, all fluently. Another German, Spanish and English. They come here and take a foreign language, while they are building their English. It seems like once you learn a second language the third is even easier. I had a friend who spoke English and Spanish and picked up Portuguese in less than a year. Now he speaks three languages well. The key to retention is practice. If you never use all three, you won't retain them.