Trilingual family here as well - Thai and French. My boy is turning two in the next couple of months. We have been using also OPOL but sometimes both of us throw some English, just out of habit (because we're speaking together in English). He goes to bilingual nursery - English and French. Although we're changing him to another nursery which is totally French from September.
He is actually an early speaker. He understands/speaks Thai the most because it's his mommy's language, then English second (from nursery and playgroups), then French. But I would way his English and French is very close. I have known lots of trilingual families or even quadrilingual, some of them speak later, some speaks early. I think it's one of those things like crawling or walking - it's either you get it early or not. No need to rush or worry about. If your child is going to speak three languages at the end, a little slow start is worth it I think.
Currently he's actually translating everything. If he says a word in Thai, he will say again in English and French. It is hilarious. Although he won't do this if he blurt out a sentence.
He also tries to different language with different people. If he meets someone new, starting speaking Thai to the person, the person doesn't understand, he will switch to English, if still doesn't understand then he'll try French. He already has a concept that there's three ways of calling one thing and that not everyone understands all of these three ways.
I think it is a brilliant gift you could give to your child. I also have read somewhere that being a trillingual helps develop the brain in a speacial way. Well don't quote me on that but you can always consult Mr. Google.