Who's under this 'we' category you speak of?
As for waterbenders bending water in all 3 phases, it's pretty obvious. Water is a specific substance. It's molecular formula is H2O no matter if it's a gas, liquid, or solid. Earth on the other hand is no specific substance. And once again you seem to be falsely assuming earth has multiple phases like a single molecule, but in order to be earth it must be _____. Can you guess what's supposed to be in the blank class? If you guessed liquid or gas, you phail.
If they were called rockbenders, then they'd be able to bend rock in a solid or liquid form because the definition of a rock is a solid or liquid material made of minerals. It's crucial that you guys understand the difference between minerals, rocks, and earth. They are NOT synonyms.
If we go by the logic that earthbenders should be able to bend any rock regardless of phase, then the same should be said for airbenders being able to bend the atoms in the air reguardless of phase. Oxygen and nitrogen can be made into solids and liquids but we never see that happen.
That's because in order to be air, a substance must be a gas.
Just like in order to be earth, a substance must be solid and made of minerals.
In order to be water, a substance must be composed of H2O.
Fire in purest form is heat. So anything that generates it's own heat would fall under firebending. I'd be willing to bet an uber duber fire master could do their own version of bloodbending by controlling the heat inside someone else's body and cook them like a microwave. Sadly, I'll never see that happen.