But isn't air already there like water and earth

Yes, airbenders do not create air, they simply manipulate the air around them. For firebenders, we have seen that it is very hard for them to bend in cold environments, and so I think firebending in the cold of open space would also be near impossible.

In space there is no air or very little air as we know it. And fire only spreads when there is oxygen no air no oxygen. So both are a no.

And what HoT said is true as well ofc.

Yes, firebenders can' bend in space because there is no oxygen there. Airbenders bend the air around them