Classica I is the most firm for infants/toddlers. Classica II is less firm but still firm. I had the Classica I for my first baby and I am so glad that I purchased the foam mattress. It is so much lighter, therefore, most definitely easier to change the sheets. Those mattresses are hard to get out of the crib to change since they are a tight fit. For the child on the way, we ordered the Classica II and we now put that in our 22 month olds bed and will re-use the Classica I for the baby. I would totally get the foam mattress and the Classica's are best rated of them. Here's Colgate's website https://www.colgatekids.com/products.php?id=2
Re: what's the diff between Colgate Classica I and III?