I really liked this 0% SR template from Chaos: http://warlight.net/MultiPlayer?TournamentID=3887
Eastern Asia is another very good choice, allows large map specialists to shine more.
Rise of Rome is a very popular 3v3 template, but maybe you want to avoid macrobonuses?
I've played 3v3 on New York - Big before and had a good time. (Also has macrobonuses but they tend not to come into play)
As much as I hate Poland and random distro, it's been used in competitions before, and it's a well known template.