I could've spent 3 weeks in London and still not have seen or done everything I'd wanted to ... and this was 27 years ago!
However, you can pack a lot into 3 days if you plan them well!
Public transport is very easy to use, and it's a very walkable city - I loved wandering around Covent Garden (wasn't supposed to be on my own, but ... introverts are kinda sneaky; I just made sure the chaperones saw my face every now and again), for example, but I also adored the Tower of London tour (was standard-with-admission Yeoman Warders overview of the history of the site, nothing extra, although there are extra tours too) and seeing the Crown Jewels. (And realizing exactly how tiny Queen Victoria had been - one of her formal gowns, I've forgotten if it was her wedding gown or her coronation gown, was on display in the queuing area - was kind of interesting.)
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Liz
DD (3/2010)
"Make mistakes! Get messy!" - Miss Frizzle