You can probably start training with 16 week out (i.e. start at end-June) so long as you had a base. By base, I mean being able to jog for about 30 minutes or more without stopping.
There are plenty of schedules around – a good one to look at to get an idea of what would be required is here (although it is for 4-5 days a week of training):
http://www.runnersworld.co.uk/news/article.asp?UAN=108
You could replace either of the Monday and Friday runs with another sport.
I know once you enter for the Auckland marathon they will email you a schedule each week from about 12 weeks out, depending on your level, one of which is a 3-day schedule. If you only want to run 3 days a week I’d recommend getting a base, and then adding at least one other day doing a cross-training activity (swim, netball, whatever).