I'd go with your latest date 'officially' Anna, cos then, as the others have said, you'll have more room to go over. In your head though, be prepared to go early (especially having your 3 so close together).

From my observations, some people are long bakers and some are short bakers too. Some people consistenly go over, have their babies at 41 weeks, and others have never made full term. So I'd go with your track record a bit as well - for example, Riv was 5 or so days early, Si was 2 days, so I'd be prepared for a third baby to come earlier as well.