They can last for more than 3 weeks, but as your nails grow, you get an unvarnished bit, which isn't very attractive. (Unvarnished nails aren't unattractive, but an unvarnished stripe on varnished nails is.)
If I were you, I would have them done again between events 2 and 3.
Three weekends is only just over two weeks if you get them done on the first Friday. I've only had them a few times, but regrowth has started to be noticeable after a fortnight, so maybe go for a light colour or French where it won't be so noticeable.