I've just realised that I've been having a different problem, without realising it, which may explain the 3 chains being too long!
I've been following the pattern as though it is in English notation, when it is actually in American. So I've been doing an English double crochet, when I should be doing an American Double, which would be an English treble. Now I'm following the pattern the correct way, the 3 chains make a lot more sense! It also explains why my heart seemed to be in different proportions to the pictures!