We're not trilingual, but we are bilingual.
We saw no noticeable language delay, although I will day neither of our DC were particularly early speakers. They were very average, with about 20-30 clear words at 18 months and short 2-3 word sentences by 2.
I would say a couple of things:
- To succeed, you need to provide three things: quantity of exposure, consistency of exposure and need to speak all the languages.
So, for quantity, you need to make sure your LO are getting a couple of hours per day at least, I would say.
For consistency, it basically means that a one or two week holiday (or even a two month stint) in the minority language country, and then nothing the rest of the year will not work. It needs to be consistent over time, building gradually and incrementally.
And for need, this is a tricky one, because if a child knows their parents can speak the majority language, their motivation to maintain the minority is reduced. So, minority language monolingual grandparents, cousins, aunts etc. can be a great advantage.
- My DC both started school with only the most rudimentary knowledge of the majority language.....and despite what every told us, it did take a good while for them to get to grips with the majority language. Everyone told us they would be fluent in a few weeks, which they most certainly were not. I think the language issue did complicate the first few weeks of school, settling in etc. And I would rather have had them slightly more au fait with the majority language before school, but that's not what happened for us. My DC 1 who has done three years of school is still slightly behind her monolingual peers in the majority language and yet she is advanced for her age in the minority language (English, which is fascinating, as she has never lived in an English-speaking country). However, her teachers are not concerned, she is doing extremely well at school, but there's no denying that her vocab and fluency are behind her English.
- Be prepared to put in loads of effort to maintain all three languages. You will hear both positive ('oh, how lucky your children are, they just pick up these languages like sponges') and negative ('how will they keep up at school?' and 'oh, they have such a cute foreign accent when they speak x language' and 'aren't you worried they are going to get mixed up?' etc.)
Ignore those giving advice with no experience. Be prepared for a long, hard concerted effort on your part to get all three language established and then maintained.
Good luck.