Phobos has submitted a Betaflight PR to update the ELRS SPI receiver code to be compatible with TXs running ELRSv3!
https://github.com/betaflight/betaflight/pull/11783

Note that you must be running ELRS v3.0.0-RC2 on your TX to work with this code currently, and only ELRS v2 RF modes are supported (eg no FLRC).  Here is the description from the PR:

This PR contains all changes required to be able to bind and use SPI ExpressLRS receivers with 3.0-RC2 ELRS transmitters.
All current modes work as expected, there are a few extra features inherited from 3.0 ExpressLRS TX code changes.
Modes and new features that work:

• LoRa 500 / 250 / 150 / 50Hz modes with telemetry and MSP.
  
• Increased precision over 2.x modes - 25% better stick precision.
  
• New SNR based Dynamic Power support.
  
• Race and STD telemetry modes.
  
• VTX admin fixes thanks to @CapnBry .
  
• Fixes for 1/2, 1/4, 1/8 telemetry ratio thanks to @CapnBry .
  

Things that are NOT implemented and will NOT work:

• FLRC 1000 / 500 modes.
  
• DVDA 500 / 250 modes.
  
• FullRes modes.
  
• LBT.
  
• Betaflight config over WiFi (as there is no ESP onboard ? ).
  

This PR should be merged only after ExpressLRS 3.0 stable is released.
Please test with caution and on the bench with props off first.

If you want to try it out, you can flash the compiled HEX (attached, or available in the PR above) and then apply your saved diff (you saved a diff, right?   ). 

Does this mean that the current onboard SPI ELRS RX's will still be viable for now?

(20-Aug-2022, 02:29 AM)brettbrandon Wrote: Does this mean that the current onboard SPI ELRS RX's will still be viable for now?

For as long as ELRSv3 is around (and we're still in the RC stage), yes. Essentially this means you can use a TX flashed with ELRSv3 with a SPI ELRS RX and avoid having to keep a TX on ELRSv2. 

That said, there should be several new FCs on the market soon with on-board serial based RXs which will operate like stand-alone ELRS receivers and will support all of the new features of ELRSv3+.

That would be really cool. It seems at the minute, all you can get is Whoop AIO boards with onboard ELRS. Would be nice to see some bigger 20-50A AIO boards with ELRS built in, so we can put them in bigger drones without needing to solder in external boards.