On Sunday, we went to visit our good friends who are also trilingual. He is Brazilian and she is Russian. They speak English to each other. When their first child was born, dad took the decision to speak English to him as he was worried that he wouldn't speak English well otherwise, and in order to avoid confusing him. Mum, on the other hand, spoke Russian.
This arrangement is still in place 10 years and two other children down the line, except that, progressively, Russian seems to be decreasing. During our visit, over 95% of the family conversation we heard was in English, with only a few small comments from mum in Russian. So, on first impressions, the family's trilingualism seems to be declining.
However, interestingly, nowadays mum speaks pretty good Portuguese (though dad's Russian has never really taken off!), so she is becoming trilingual although her children may not be at the moment. And they generally travel once a year to both Brazil and Russia and the children love these holidays. So they function well in those environments despite their lack of fluency and they have positive associations with these countries, which could provide a foundation for developing a later interest in the languages.
In addition, we were interested to see that, while our children may speak better Russian than theirs, in some respects their children have a far greater Russian influence in their lives: for instance, their Russian grandmother had just visited for a week from Russia and so their kitchen was full of traditional Russian foods (although the children didn't particularly want to eat them!). On the other hand, all our grandparents are here in London and are therefore fairly 'anglified' or, more accurately, internationalised when it comes to food: I can't remember the last time my mother-in-law cooked a Russian dish.
They have also recently moved house and have chosen a sparkling new-build. This is definitely a Brazilian influence: no cravings for period features in this family!
'Triculturalism' can take so many different forms - it will be interesting to see how this family continues to evolve: will language suddenly spring out of these influences at some point?