Our 3yo dd & 21mo twins were/are raised trilingual just like our 5yo, however we now live in Germany away from my dad (who was the main Russian speaker in their daily lives). They skype my dad & brother every few days and they're (DC) spoken to in Russian.
I don't know if they are or not but from the few conversations I've had with them (in Russian) it's like they've gotten sloppy, excluding our 5yo, compared to how well they spoke in Russian back in England.
Can they lose a language? How do I stop it or improve their language while trying not change the main home dynamic (I speak English to them, DH in German)?
They most definitely can lose a language! If you want their Russian to progress in line with their German and English you need to organise exposure. How about a Russian (or more probably Ukrainian) nanny?
Definitely. Friends tried to raise German/Turkish/English with Turkish the less often heard language. Their children virtually can't speak it at all.
I think you need a lot of input to keep a language alive. I learned Dutch by having lots of ?Summer holidays there, watching Dutch TV but as soon as it stopped, I was 19, I lost the knowledge. I think I can go back and get it out of the depth of my memory but it will be a chore.
If you think Russian is necessary try to see if you have a Russian community around for playgroups etc. depending on the area in Germany you should have pockets of German-Russians but they normally only speak to to a close-knitted community.
There is a lot of evidence to suggest that languages are not fixed in a child's brain until the age of about 11 or 12. They pick them up quickly but lose them equally quickly if exposure is not maintained. I know of one child who completed primary school in a foreign language - performing on a par with his peers - but completely lost it when the family returned to UK and exposure was lost. If you want to retain the Russian - which will probably develop more slowly than the languages to which you children are exposed on a more regular basis - you will need to ensure a variety of inputs. Russian language DVDs, music, reading books to them in Russian if you can. There are large Russian speaking communities in most German cities. Can you get a Russian speaking student to come a couple of times a week to play with them? A Russian speaking babysitter? Cleaner? Can they spend time with a Russian speaking grandparent in the holidays? It is an uphill struggle but definitely worth it!