we sent ds1 to 3 year old kinder. he has just finished 4 yo kinder and will start prep next year. he really liked 3 yo kinder as he met lots of new friends.
we have decided not to send ds2 to 3yo for next year. he doesnt really need it as he gets heaps of socialisation (mainly with the siblings of the friends that ds1 met at kinder!). it is not compulsory and we feel that ds2 will benefit from an extra year at home - i guess it depends on the individual kid though? he will do 4yo kinder the following year. you don't have to do it if you dont want to. we found that there were quite a few kids who came in at the start of ds1's 4 yo year who hadnt done 3yo.