Our son will be 3 in August, but we moved to France when he was only 10mths old. He was already living in a bilingual environment prior to this (I speak to him in english and my husband in dutch) and we also had the same concern in moving to a country where he would need to pick up a 3rd language. He goes to a french créche and has no problems at all. There are times when he speaks in english to the other kids, sometimes dutch and increasingly now in French. Sometimes they understand him, sometimes not but it has no bearing on his social interaction with them - in fact, some of the french kids have now picked up on his english and they're all mixing it up and playing happily together.
What I'm now noticing is how quickly he adjusts when friends and family come to stay. He will talk to one person in dutch and without missing a beat, turn to me and say the same thing in english. At our village market, he talks to everyone in french and seems to know instinctively which language to answer in. Of course there are times when he mixes all three languages together to make a whole sentence "I'm putting op (on) mijn (my) zwembroek (swimsuit) mama, on y va! (let's go!)".
In France, the kids generally start school at 3, and initially we had opted to send him to an international school but seeing how well he's adjusted we'll happily send him off to the local french school. Coming to France and immersing him into the community as soon as possible has been a great decision. Give it a few more months and I have no doubt that my son's french will be a lot more fluent than mine! In our experience and according to a lot of friends who are living with their kids abroad; the younger they start, the better.
An aussie married to a dutchy living in France with our 2.5 year old trilingual son.