i was there for a couple of days too, you cant miss the big ben, westminster, kingham palace, tower of london, st pauls cathedral, etc etc
My advice would be to plan your trip with time. i asked people what places i couldnt miss and i also did a google search, then made a map with my route on google maps (yeah, i know it sounds silly but it will save you a lot of time, specially useful if you're gonna be there for just 3 days).
Oh and get the oyster card for the subway. It will save you A LOT of money (just ask about it when you get there, you can buy them in pretty much every major subway station). A single ticket costs like 5 pounds (yeah, stupidly high prize), while with the card you'll pay like 1, and it works for buses too.