game-and-watch-patch VS game-and-watch-zelda3

That being said, if you want to triple boot the device, Brian's patch explains how to do that here. This would probably be the best option since you want to keep the original firmware.

Which part exactly are you not sure about, flashing the firmwares or installing the chip? The flashing part is actually really simple, The commands are fully laid out here

If you plan to keep the stock original games you'll only have around 1MB of free space left on the Zelda unit. You can help free up more space as mentioned here.

All u need is already on the GitHub of each thing u need. The backup thingy is the same for both Mario and Zelda. The retro go setup (and CFW to have both the stock firmware and retro go at once) have a section for Zelda at the end too

I note that I am on a virtual machine with Ubuntu 22.04 and a Stlink Nucleo-F446RE. NOTE: My username on Ubuntu is ed and I have installed in the Documents folder the arm-gcc-none-eabi files. 1. !! be sure to have made a backup of your G&W with your original chip and to have done at least the first 4 steps !!. See ghidraninja's Git -> https://github.com/ghidraninja/game-and-watch-backup 2. Make a git clone of the G&W Patch and G&W Retro Go pages -> https://github.com/BrianPugh/game-and-watch-patch -> https://github.com/kbeckmann/game-and-watch-retro-go 3. It may be useful to install these dependencies: -> pip3 install -r requirements.txt -> make download_sdk

I think you can search youtube for the tutorials, but don’t follow them exactly, because most of them do not quite know how to do soldering XD. If you have a hot air gun it will be much easier, but it is still doable with solder iron. I suggest you to desolder the speaker from the board and remove the board from the case before doing anything. You can test and flash the ROM without reconnecting the board to display. You can flash the ROM with a pi or st-link adapter. I’m using a st-link adapter and I suggest to do so. It is a lot more stable and faster than using pi. Here are all the useful links of software that I’ve used. Backup and Unlock the G&W, do it before any hardware mod https://github.com/ghidraninja/game-and-watch-backup Patch original game so launch retro-go with SET+Left https://github.com/BrianPugh/game-and-watch-patch Orignal Retrogo from G&W https://github.com/kbeckmann/game-and-watch-retro-go Forked version that support cover art (I’m using this) https://github.com/olderzeus/game-and-watch-retro-go Convert MAME G&W games to Retrogo, you can download G&W games from any MAME archive. And google “Game and watch artwork” for the artkworks, you should able to find a google drive sharing folder https://github.com/bzhxx/LCD-Game-Shrinker

The repo to compile the ALTTP ROM for the G&W can be found here. The stock firmware dual boot repo has information on how to triple boot the unit. But for testing (and when I found the bug he fixed for everyone) I just had flashed only ALTTP to the unit.