Successful cross-flashing depends very much on the compatibility of the underlying hardware of the two devices. i.e. The board of the receiving device must be the same as the board in the hardware which is usually flashed with the donor firmware. In conventional Garmin devices this is determined by them having the same "PCB P/N" (read this thread to understand more fully: [Only registered and activated users can see links. ]).
(TLDR)
Points to note:
  • Matching PCB P/Ns does not guarantee a successful conversion, however non-match of PCB P/Ns will result in failure and nearly always a bricked device. That may be a fatal bricking in the literal sense with no way to recover the device using any software method.
  • Flashing a different fw will always fail if the basic types and region layouts of the original and donor firmwares/devices differ markedly. Flashing a different fw results in the region layout of the receiving device being changed to conform and if that's beyond the physical limits of the flash memory failure is certain. One way to experiment with relative safety (in conventional devices) is to use a 'hybrid' fw made up of the boot.bin (ramloader) of the receiving devices original fw combined with the other components from the donor fw and retain the device's HWID. Then if unsuccessful it's usually possible to flash back because the region layout is unchanged. A successful hybrid flash usually, but not always, means a complete flash will also work and with changed layout and changed HWID.
  • Even if a complete conversion is technically successful there may be features in the donor fw that require the presence of certain other hardware which may be absent in the device being flashed, or vice versa, reliance on the fw having the ability to utilize hardware but such code is not written into the donor fw.

Obvious differences between the two firmwares in question:
  • Fenix 3HR's fw although not encrypted has only one section consisting of the main system software "fw_all.bin" and has no "boot.bin" component (the ramloader). Usually (in conventional devices) that ramloader is flashed to a virtual region (decimal 12, hex 0C) and then that virtual region in turn flashes two physical regions, rgn 5 with the bootloader and rgn 43 with the X-loader and the actual boot process is started from there. It seems that the 3HR's ram is somehow self-loading and maybe contains a bootloader itself but that's conjecture, i really don't know how it boots up at all.
  • Fenix 6SPro/ProSolar is firmly encrypted. Like a conventional fw (but unlike the 3HR's fw) it does have two sections however the first section doesn't have the conventional ramloader for rgn12, instead it has a bootloader for direct flashing to rgn 5. It may refuse to accept a non-encrypted fw flashed by RGN anyway.


Short version: Likely potential or even certain problems may result even in the event that the 3HR and 6SPro are indeed basically the same hardware, so you'll be proceeding at considerable risk.