Your three year old probably can't use it. Current voice recognition - Siri, kinect, et al - all seem to struggle with child voices. Maybe something they've expressly worked on for this 'home assistant' scenario, but I don't hold out much hope. It's a general pattern that early releases of human-interaction tech tend to optimize for 50th percentile western males.
When one of my daughters was 3 someone handed an iphone with one of those apps with an animal that repeats things back. She said something with the word "color" in it, but it came out "cala."
It repeated it back (incorrectly) and then she got stuck in a loop of saying "not cala, cala!" and it repeating it back, getting more frustrated each time.
I'm guessing that was Talking Tom (the cat). My 2 year old daughter has gotten into similar loops with it several times. Usually though, it's just along the lines of, "No, YOU!"
Children generally don't enunciate as clearly as adults. Their speech is more difficult for a computer (or a human who doesn't know the language well!) to understand.
