I don't think Sora stuff has trim (maybe in the big ring only), and at least the Sora FD I've used didn't have an adjusting barrel, either. However, if the rub occurs only the two big sprockets, maybe you shouldn't be using them when in the middle ring? Or, you could try to loosen the cable just a bit, but then you might experience rub way too early when going towards the small sprockets.