Have a Spektrum DX3.0 transmitter
-can receivers from different manufacturers be used with the Spektrum transmitter (eg see Tactic has a cheap 3ch 2.4GHz radio)
-if need to keep to Spektrum, can the air receivers be used?

They have to be Spektrum RX's. I think there is a coding difference between their surface and air rx's. But try it won't hurt.

The surface and aircraft RXs are not interchangeable.

It's not advicable to swap with different manufacturer receiver. You might bush your servos. If you want to try do it at own risk & manufacturer does not support the config per manual.

Other than that surface & air is totally different.

Think about it, why would they make air and surface rcvrs compatible? You could be the one causing interference and someones uber expensive plane to crash.

Even with FM, air and surface were kept diff by using diff channels - 72 for air vs 27 and 75 for surface.