You will be hard pressed to find snowboarding boots for a kid that young. And it's kind of hard to tell if they fit right anyway since kids have a harder time understanding and explaining how their foot feels in the boot.
Honestly, I would recommend starting your boy off skiing if he's only 3. Is it possible for a 3 year old to snowboard, yes, but it will be harder and most likely not as fun for him. I taught snowboarding for 4 years and we wouldn't even take kids under 6 unless they paid for a private lesson for snowboarding. The youngest I ever taught was 5 and he had a hard time picking up the mechanics. He can be placed in a ski school lesson a lot easier and enjoy himself much more if he starts out skiing. Plus, if he can understand the concepts and execute the motions and increase his balancing abilities from skiing it will help him to transition to snowboarding when he is ready. Also, most places will have rental ski equipment that will actually fit him.
If you really want him to learn snowboarding and not have any affiliation with "the dark side", then that's totally up to you, and I'm not trying to say that it's a bad idea or anything. But that's just my advice and recommendation. And someone else suggested on another thread that you could just use his regular snowboots for snowboarding, since he probably won't be doing any major carving or anything for awhile.
Don't just live life on the edge, jump off it!