I finally got it flashed!
The steps confused me a bit, not sure if it is straightforward for everything else, but simplified is:
- as per https://wiki.radxa.com/Rock3/install/rockchip-flash-tools, install the driver from https://dl.radxa.com/tools/windows/RKDevTool_Release_v2.86.zip
1a. extract, run the DriverInstall.exe then click to install the driver
1b. REBOOT! - MaskROM mode (UGH)
2a. move the TuringPi jumper from boot to flash
2b. connect USB microB cable
2c. HOLD the tiny MaskROM button WHILE plugging in the TuringPi power - flashing debian from https://github.com/radxa-build/radxa-cm3-io/releases/
3a. download https://dl.radxa.com/rock3/images/loader/rock-3a/rk356x_spl_loader_ddr1056_v1.10.111.bin
3b. download https://github.com/radxa-build/radxa-cm3-io/releases/download/b27/radxa-cm3-io_debian_bullseye_xfce_b27.img.xz and extract it
3c. run RKDevTool.exe
3d. uncheck all but the first two lines
3e. double-click the second line Name column and type ‘Image’
3f. in the last column, click the blank area in line 1 to select the rk356x_spl_loader_ddr1056_v1.10.111.bin file
3g. in the last column of the second line, click the blank area to select the radxa-cm3-io_debian_bullseye_xfce_b27.img file
3h. click Download (yes, it really is flash, not download)
After it is done writing, it will reboot the Radxa CM3S, but nothing will be usable on the TuringPi until the jumper is moved back from flash to boot and restarted.