Sandra, Around where you are based in Greece the simplest way of telling these apart is where you find them.
For starters, you won't find Cyclamen persicum growing wild in mainland Greece. (Very occasionally you may come across it in ancient graveyards, descended from ones people have planted there in the past.) And as John says the wild ones flower in spring (except for some populations in Israel), but the other two species flower in autumn.
The plant which flowers abundantly on the hillsides around and above your Greek house, normally either in open ground or in dry phrygana, is Cyclamen graecum.
By contrast, Cyclamen hederifolium tends to grow in more shaded, rather damper or wooded places - little sheltered valleys etc; for instance, in your area it's common around Milia and Castania.
As John says, once you have seen quite a few plants, you can tell them apart at a glance, though it is quite difficult to put the differences into words. Until the leaves have got really weathered, the leaves of Cyclamen graecum often tend to have a rather velvety look, as opposed to the smoother and glossier look of C. hederifolium. Cyclamen hederifolium leaves can get quite a lot bigger than those of C. graecum - sometimes as big as your hand or even larger; even just finger-length would be pretty big for C. graecum.
Looking at the pictures in this thread, comparing C. graecum and C. hederifolium leaf patterns, should help too.
I have to say that it is really difficult to tell the flowers of these two species apart, but around your area, at least up to 500 metres or so, you have the bonus that the populations of C. hederifolium are often gently scented, whereas C. graecum never is.
