Apparently, this question comes up often on YouTube channels maintained by polyglots.
For example, in the six-minute video Maintaining multiple languages Vladimir Skultety explained how he maintained the languages that were important to him at that time (Chinese, Italian, Russian and German; the video dates from November 2016):
- Try to do things that don't require his full attention, such as listening to a German podcast while at the gym, listening to an Italian talk show while cleaning up his room, listening to something in Chinese while jogging.
- Since this can get boring, it is important to vary your activities: reading books (good for developing vocabulary and language structures); simultaneously translating in his head, e.g. while listening to a Russian talk show; chatting with people through voice chat (which does not even feel like learning).
- According to Vladimir Skultety, the most important thing is to combine activities in order to use your time more efficiently.
- He tries not to neglect a language for more than three to four days, because otherwise his level would start to go down.
At the 2014 Polyglot Conference, Alex Rawlings gave a 30-minute talk about How to learn and maintain multiple foreign languages:
- There is no single way to achieve this, so he presents his own ideas and experiences.
- The basics are simple: (1) learn the language; (2) speak the language; (3) keep speaking the language.
Repeat the process for the next language.
- On a more serious level, to learn the language, (1) you need to set realistic goals; you cannot rush language learning; (2) find learning materials and/or a teacher; (3) decide how much time you can allocate to the language; Alex Rawlings recommends one hour per day split into 15-minute chunks (you need to find out what works for you); (4) review and check what you know; if your learning materials contain tests, use them; (5) make changes to your schedule if things aren't working (this applies to learning materials, teachers, etc.).
- To speak the language, (1) you need to locate resources (someone you can speak with, not necessarily a native speaker); (2) find a teacher who knows hot to correct your errors (e.g. just speak for 15 minutes and list the errors later); (3) find authentic materials in the target language that contain cultural information and that contain constructions and expressions that don't exist in your native language.
The above is mainly about learning a language, but the question "how to keep speaking a language" is about maintaining a language.
- You need to integrate the language into your life, e.g. by visiting the country why the language is spoken, spending time in an immersed environment (a course or a job).
- Try "remote immersion" by reducing your use of L1 and think in your target language instead.
- Read, watch TV and listen to content exclusively in your target languages. When Alex Rawlings was learning Afrikaans, he started reading books in Afrikaans, watched videos in Afrikaans and listened to the news in Afrikaans, all as if he were in South Africa. This took him to an intermediate fluence.
- However, it is difficult to guarantee that your "remote immersion" is authentic, that it represents the language as it is really spoken. This is easier to achieve with English, because it is everywhere, than with smaller languages such as Hungarian.
At the end of his talk, he discusses how much we have to sacrifice to become a polyglot.
- He integrates foreign languages into his life. At the time of the conference, he was treating his Hungarian grammar book as a sudoku book (he dipped into it when he had 20 minutes of spare time), he was reading exclusively in German because he enjoys reading in German and it provides a lot of language input. This way, he does not have the feeling that he needs to sacrifice a lot of his spare time.
- One of the audience members integrated Russian into her life by listening to music, watching films and by changing the language of her devices to Russian. She also labelled things around her for which she did not know the words.
In the 10-minute video Maintaining multiple languages at the same time | Which to learn, schedules, pronunciation etc, Lindie Botes shares her views:
- It is a matter of time and motivation. You need to be prepared to deal with frustration.
- It works better with language that are not so easily confused.
- Become conversational in a language before you pick up the next language. This includes getting the pronunciation right.
- Make a schedule if that works for you. Lindie doesn't use a schedule but learns more haphazardly. If you make a schedule, priorise your languages and make your schedule accordingly.
- Set realistic, bite-size goals.
- In order to make sure that you use your target languages, you need to integrate them into your life, e.g. by maintaining a diary in a foreign language, listening to music, watching TV, etcetera.
In the nine-minute video How does hyperpolyglot Luca Lampariello maintain so many languages? (November 2013), Luca Lampariello explains his approach to Conor Clyne:
- He decided to embrace a certain lifestyle that allows him to learn and maintain multiple foreign languages. When he was still living in Rome, he lived his life "through and with foreign people".
- If you live in a country with fewer foreigners, you can still talk to foreigners over the internet. In order to find people online, he uses his YouTube channel, but there are many other websites where you can find people who are willing to do a language exchange. For example, italki, Sharedtalk (which closed down in 2015, but now there is SharedTalk on Biligua.io), "language exchange" (that name is ambiguous, see e.g. language.exchange and LanguageExchange.net) and Conversation Exchange. However, you should be a bit selective in your exchange partners; you will learn most when you talk to people you like talking to.
- Conor Clyne makes sure he has some exposure to each of his targets languages every day. Luca Lampariello also says that you need to tackle every language almost every day. (Luca teaches languages online, so he needs to use several foreign languages every day.)
In the hour-long talk Maintaining Multiple Languages At Once at the Polyglot Gathering in Berlin in 2014, hyperpolyglot Richard Simcott talks about how to maintain five or more languages. He says that ten foreign languages is a critical number. One of the important things he mentions, like other polyglots, is integrating those languages into your life. Maintaining five or more languages is a life choice, since time will impinge on what you can do.
Although none of the above videos are specifically about maintaining three foreign languages at level B2 or higher, integrating those languages into your life is a common theme, regardless of the level you aim for.