Hi! Welcome to YT. Have you already purchased the puppy? If so, you will have to make extra provisions with regard to your 2 younger children being around the pup. My son was 5 when we got Chewy and he was totally prepped about what it meant to have a small dog and given the rules, no touch, no riding, etc... He got it and was perfectly cool with it all. Your younger ones won't understand and I would definitely not leave the dog unsupervised with your 3 year old. There was a story on here recently where a lady had her 2 year old running in the yard with her yorkie. The 2 year old fell on the yorkie and unfortunately, the pup died. It can be that quick.
I think actually having the pup around your 3 year old will help. All it is is taking the fear away. You could do that by giving him a job to take care of the pup - maybe letting the pup lick the nutrical off his fingers or letting him help pour the food.
Good luck with your new pup and just make sure to be careful
