I haven't posted for a while as I have been busy trying to write my dissertation, but I thought I'd post with a few thoughts on things that have helped me or others I've spoken to on the path to trilingualism.
1. The consistency question
We have certainly found that being consistent and only speaking our non-community languages to our kids has helped tremendously. After an initial effort when Dragonfly was born as I certainly hadn't been speaking French much for about 10 years, it has become second nature.
However, I'd say it's not necessarily the only approach that might work: it all depends on other factors. For instance, if you have grandparents around a lot, or spend extended periods in the relevant country, the children might get enough exposure to the languages from that.
2. Having a language in common
Many couples in trilingual families speak the community language to each other as that's the only language they share. We tend to do that too, but can also speak Russian to each other and that does help: if we make an effort to speak Russian at home it drastically increases the amount of Russian the kids hear. We also occasionally have a 'Russian breakfast' or 'Russian car journey' when we all speak Russian as that is the language the little bugs hear the least.
It also helps that we both understand what's going on when we each speak our respective languages to the kids - we don't have to translate everything or feel awkward that the other parent is left out of a conversation although we do sound like a family of mad people!
On the other hand, a friend of mind pointed out that, if you don't understand your partner's language you don't have to pretend you don't understand when the child addresses you in that language: you really don't. So this can make a 'strict' approach to utterances in the 'wrong' language come naturally.
3. Modern media
We are so lucky to be able to access media from all over the world at the touch of a button these days. Youtube is full of cartoons and films in any country, you can get apps and games in all sorts of languages, you can order books from most places to be delivered to your doorstep. Several parents I have spoken to recently do as we do and use these 'treats' as tools in their language, for instance allowing TV but only in one of the non-community languages.
4. Fun fun fun!
Related to the above, we find that the more fun the kids have with their languages, the better. The other day, they were watching a slapstick Soviet comedy from their dad's childhood and they were delighted. Both Dragonfly and Bee love singing all the French songs from my childhood (as well as some additional ones we've discovered on various family CDs). If we go on holiday to France or Russia, we take them to as many fun things that involve language as possible: this could be the theatre, puppet shows, the circus, ski lessons, museum talks, arts and crafts activities etc. If we're in the car, we'll listen to hours of audiobooks in all three languages - it makes long journeys so much easier! Bee loves games so we play lots of games with him and many of these involve a lot of talking. Bee has also discovered football and as this is definitely his dad's thing and not mine, he has to speak to his dad in Russian to discuss the latest scores and request that they watch clips of the latest games.
5. Complementary schools and other sources
At the moment, Dragonfly and Bee go to an after school Russian club and that has helped their Russian along quite a lot. They've also learnt some classic Russian kids' songs and a bit of Russian folk dancing which all builds up their Russian links.
However, we used to attend another one locally, and it was a drag! 4 hours every Saturday involving science, mathematics etc. although Dragonfly was only 4 years old. It was such a chore that it was turning her against Russian and we dropped it. Partly for this reason, we wouldn't add a French club at this stage: school overload is never great for kids' motivation!
We've also briefly thought about sending the little bugs to a French or Russian school for their main school, but the dilemma is that it would give a very strong weight to one language, which doesn't seem right to us. If one language is to dominate, we feel it is 'fairer' that it is the community language.