E.S.
My girls go to preschool starting at 2 years old. For 2 year old, 3 year old and 4 year old preschool they have gone M-W-F from 9a-12p. My 3 year old right now has a late birthday (November) and she actually has been in the 2 year old preschool room for 1 1/2 years. I really feel that they benefit from it, regardless of whether they are shy and introverted or outgoing and extroverted - I've had both. When she started going last year, she barely said anything. I'm not kidding that within a month, she was talking up a storm. My first one went into that 2 year old room, said goodbye, and started playing. My second one kicked and screamed and clung to my leg. Within a week, she was saying goodbye and playing and asking why she couldn't go to school when it was an off day. I don't think the curriculum is any different whether they go two days or three days, but if the question is whether 3 days is too much - you'd be surprised, the answer is no. They get an extra day of crafts/playtime and all the kids love it. Be prepared for the first week of school crying, screaming, pleading, etc. The teachers are prepared for it and they will tell you too the best thing to do is just leave. Give them a quick kiss, tell them you will be back when school is over and leave. They won't be crying when you come back I guarantee it.