It's not only about the ohms, but also about sensitivity. The K3003 are 8 ohms and have high 125db sensitivity. The H3 on the other hand are 40 ohms with rather low 107db sensitivity. Which means they're unlikely to hiss from most sources, but require a decent amount of driving power.
Don't get me wrong, I can still drive them to plenty of volume straight from my DAPs. It's just that they sound a little bit better and more controlled from more powerful sources.