My understanding of 3yo kinder is that they mainly focus on learning socialisation and thing around taking turns, following instructions, listening tot he teacher etc. There is very little in the way of "school" type learning so if you can get a couple of days in a childcare centre that has a 3yo program then that would be fairly much the same. If you can get one in an area where it is close to the 4yo kinder or school that you hope to send your child to then they will meet other children who will also probably attend them.

I have DD1 in childcare 3 days a week and she will be attending the 4yo kinder with these children and I know that a majority will be attending the local primary school with her. This is an advantage as it means that they know people when they start school and it isn't unfamiliar to them.