How long has he been at nursery?
What jumps out at me from your post is that the nursery language, Catalan, is not the same as either his mother's language or his father's language. I think labelling either Spanish or Catalan as the "community language" is misleading at this stage, because a toddler doesn't really have a community outside of family and daycare. That would mean that your son's native languages (that he has learned from his parents) are English and Spanish, and when he started at a Catalan speaking nursery, he was being introduced to a third language he did not speak yet. How much he understands at nursery will depend a lot on how long he has been there if this is his first significant exposure to Catalan.
The nursery will probably have a lot of experience with children who are growing up speaking Catalan and Spanish, but maybe not much experience of children who don't speak Catalan at all when they start nursery. For this reason they might be assuming that your son has a developmental delay because they are used to working with bilingual children and can see that your son isn't like other bilingual children, when the reality is that of course he isn't like them because he's actually bilingual in English and Spanish, and learning Catalan at nursery. He isn't trilingual at all. He's bilingual in two languages, neither of which are the nursery language. But I would expect that if he stays in a Catalan speaking environment at nursery and school, he will become trilingual fairly soon and will never remember a time when he didn't speak all three languages.
Is it actually a problem if he is assessed as having a developmental delay that he doesn't actually have? It seems to me like the worst that can happen is that he is referred for SALT he doesn't actually need and eventually the therapist says, "he doesn't need this, he's fine".
Whatever you do, don't stop speaking any of his languages to him.
For what it's worth, my bilingual 4 year old barely spoke at all before age 2 and barely spoke English at all before age 3.5. My 27 month old is really just starting to speak now, but is saying words and phrases in both languages. I don't believe either of them has any kind of developmental delay and am not worried.
I think some cultures are much quicker than others to suggest that a child has a speech delay. I remember feeling quite stressed because other women in my baby bumpers group (with children born in the same month as my son) were talking about getting their children referred for a speech delay because they weren't talking much yet, when my son was not talking at all in any language. But here in France, nobody has ever suggested that he has any kind of delay, and when he did start speaking everyone commented on how good his vocabulary was.