Hi Rachierad,
There are quite a few posts on the forum that talk about more natural and healthy choices for children and include a number of links to some very good sources as well.
You will find links to most of the better children’s threads and posts in post #2 here.
For a 3 year old probably yes (as long as it wasn’t too soft) but I would tend to go a little thicker so that as they grew into a pre-teen and a teenager it would still be suitable as well.
I personally think that a latex mattress with wool such as this (the wool is the fire barrier) is an ideal choice for a crib mattress as long as it’s firm enough and in a twin version (perhaps slightly softer but no softer than a medium) would be a great choice for an older child as well.
Phoenix