I have been trying to locate studies on language learning and younger kids (2 y.o. to 5 y.o.), but so far have only found studies that deal with L2 learning (e.g. https://sites.ualberta.ca/~jparadis/paradis_keynote_AP%20copy.pdf).
Our kids are subjected to three (four if you feel Norwegian and Swedish count as separate languages) different languages every day.
I'm concerned about progress and I suspect it might be a good idea to cull one language. I believe English can be introduced a little bit later.
My wife speaks Russian with the kids. I speak Norwegian. We use English between us. At pre-school they use the majority language (Swedish). The kids' first words were Swedish and lately they have used some Russian words too. They are noticeably lagging behind their peers when it comes to L1 language development. OTOH they seem to understand a great many words, regardless of language used.
The preschool teachers are not worried. Yet I feel progress improved when we were allotted more time (up from 15h/week to 30h/week). To me that is an indicator that more exposure to Swedish (and less Russian/English) has proven helpful.
So: L3 and L4 learning for late talkers -- yay or nay?