Ya... that's a tough one.
When I was young, if I saw any animal die, I'd burst into tears. I personally think 3 is too young.

However, if you were lucky enough to see a snake eating in the wild, I think that might be different. I have seen this a few times in my life, including when I was somewhere around age 6 or 8.
I think it's different because the animal has caught it's prey itself, and is not being fed by a person - and thus thhere is nothing anyone can do, it is nature.

I personally don't feed live (can you tell?) so the best I can give you is my opinion from an animal-lover's stand point
I think you should wait to see if your child is interested in the feeding process first! Just because they are interested in the hobby does not mean they want to see a snake eat.
If your child shows interest and asks to watch, that might also be a different story.

I feel like offering to show something like a live feeding should not be done, unless a child is really interested and asking to see it many times.