As Adam says, I don't think that's possible. You can preserve your apps/files/data by not flashing the userdata partition; but that's only possible if it keeps the same size (and, I guess, location). However, installing triple-boot would probably need to shrink that partition to make room for (at least) two new ones.
An alternative might be to back up all the necessary files somewhere else, flash, and then restore them. You may need root access to copy some of them, though. And I don't have a definitive answer to which files. I looked into this yesterday when I tried to back up my Gemini before flashing; but I was updating an existing OS and preserving the userdata partition, so I haven't yet tried to restore anything. From looking around the filesystem, I'd guess you should back up /data and /storage, except for /data/media/0 (which is another hardlink to /storage/emulated/0) and, if you have an external SD card, its folder in /storage. There are several cache folders which can probably be omitted, too. Also back up /system/etc/hosts if you've changed it, and /magisk if you've installed that. (Anyone have a definitive answer on what to back up?)
