ah, 24, that's the time it often starts. Aroun 26/27 was the peak of wedding invites for us (one summer we realised over an 8 week period, there was no weekend where we weren't either going to a wedding or a hen/stag do), then suddenly you run out of unmarried friends and stop giving all your money to John Lewis gift lists... 
Great timing for thinking about it, with the sales. I would say a navy blazer/jacket would probably stand you in good stead for the 3.
As well as the lovely dresses on Asos listed and coast, have a look at Boden, they have some in the clearance now.
Boobs away and not arse skimming, avoid red, black and white, while for some weddings, those are ok colours, if you want your dress to last you a range of weddings, don't get those. (white = looks like you are trying to compete with the bride, black = mourning, red = traditional colour for the groom's mistress, not much of an issue now but some older relatives might get the hump).
Tights are fine, I would think about your shoes too, lots of standing at weddings, nothing you are going to be in pain before the canapes have been eaten...
Hats/fasinators, not normally needed, although some more formal weddings you might look out of place without, worth checking. (sparkly hair accessories also works).
Do you have a clutch bag?