Jodie - yeah thats what I have found too. DS and DD1 play together most of the time and entertain each other - so I can do what I need to with DD2.. I have found it a lot easier than when I just had DS as a 19 month old and DD1 as a newborn.

The ages (if we DO decide to go for #4 - nothing is set in stone at the moment) would be:
DS - 4.5
DD1 - 3 (or almost)
DD2 - at least 19 months.

So I don't think its too unmanageable? DS would be going off to school in the next year (I think? They go to kindy or something at 5 don't they?) so I would basically have what I have at the moment.