I am Italian, married to an Iranian man and live in London. We are raising two trilingual children who are now 4 and 6 years old. I only speak Italian to them, my husband only speaks Persian, and they are learning English at nursery/school. When they were younger, I often thought that they were slightly behind other children in terms of understanding, but I used to reason that they have to learn three languages, and they would eventually catch up. And they have, of course. They still, at this age, mix the languages a bit, often turning to English, their stronger language.
My eldest is very bright, and yet she seemed to slightly lag behind at school at the beginning, particularly in literacy. Obviously she wasn’t as exposed to the English language as much as the other kids, since we don’t speak it at home with her.
On top of that, you should try not to compare your child to other children, because they really all develop at their own pace.
So, if I were you, I wouldn’t worry too much about it. What you are doing for your child is great, and you should stick with it. But you need to plan for the future, especially if you are a worrier, and try and preempt issues like this, i.e. be prepared!
I hope you find this comforting. I recently started a blog on my experience bringing up trilingual children http://trilingualfamilylife.blogspot.co.uk/.