Youngest was 3 yrs old. Went to Montessori. We paid for a full-day, 5 days a week, Montessori school to reserve the space. It was quite expensive (and we are not loaded) but I did not like the low-cost option playgroups around me.
I did not need the childcare but I liked the socialization and educational aspect of the Montessori. We used it for a few hours every day. I did not want to punish my kid by waking him early to go to school as I was already at home. Also, I did not want my kid in school for long hours because I stayed at home to be with him and my older child. We used to have a leisurely morning and he went to school when he wanted to so that he could be with his friends. He liked the school field-trips, special experiences, art and music, cultural days etc. So, I was using it very differently than traditional childcare. He still learned to read, math etc.