if you're going young, i agree. some kind of practice knife first to learn safe handling without the potential for hurting themselve. i've seen (boy scouts?) websites that sell a wooden knife kit to build as a good project and good test of hand-eye coordination readiness?). my kids were 6 and 8 at first knives. the boy had interest WAY earlier than that, it just seemed like borrowing trouble to give a 4 year old a knife. you've got a 3 year boy, so you know what a group of 3 year old boys is like - the decision-making can be very sudden and is frequently suspect.
everyone's kids are different. for
my kids, ownership of knives at 3 or 4 years of age would have been more because I wanted them to be a little woodsman with a knife and not because they were actually ready. day-to-day life - we didn't let our 4 year olds touch the guns, or use the sharp steak knives, or pour pots of boiling water into the cup-o-noodles containers. we didn't let them climb tall trees by themselves and we made them take a step back when we opened up the hot oven. not because they were bad kids (very much the opposite) but because they were only 4 years old and very bad stuff can happen to children in a heartbeat. once they got a little older and we felt like they could make smart decisions and not be so overwhelmed by toddler brains that they would do dumb in a split-second, knife ownership came into play. and it was at a little different age for each kid. Oh, I'd had their first knives for YEARS by that time, because i'm me, but it was a little later for my particular kids.
first child, probably overkill it but it worked great. i got a pocket knife and rounded/flattened (belt-sander) the sharp edge to dull. DULL. point was still a little pointy, but he would have been hard-pressed to break the skin with any part of it. it still cut butter, sandwiches, leaves, sort-of whittled, etc, but i took out the chance of him cutting himself when i wasn't watching. he was 6 and had wanted a knife like dad's for a long time by then, so i went that route and got him a real knife, just dulled it down. after about a year, started to gradually sharpen it over the course of another year or so.
second child was less fascinated. i got her a small keychain SAK (pink, with flowers on it

) when she was 8 and showed more interest. so she got a smaller but sharp knife, but it was buried amongst scissors, tweezers, etc.. it was less 'knife' and more 'useful tool, part of which is a small penknife'. sharp from the get-go b/c she was a little older.
whoa, long answer, sorry about that. hope it helped?