Ok, to answer your questions:
My phone is a onplus 3, bootloader is unlocked.

I had some free time today and fiddled a bit around, i wrote some of the things i found noteworthy into a short guide:

Note: As a new member i'm not allowed to post too many links, although i think that they're very appropriate in this post. I've confuscated them a little bit and hope i wont get banned for it

1. To get cyanogenmod to boot, one has to flash the oldest oxygenos zip by oneplus (or maybe a newer one? My internet speed is very slow in the hotel i'm at, so downloading them is a pain), found here [downloads.oneplus dot net/oneplus-3/oneplus_3_oxygenos_3.1.2/] (bottom of the site, red download button). This will get you the appropriate firmware for the old os
2. The cyanogenmod base version can be obtained at archive.org [archive dot org/download/cmarchive_snapshots/cm-13.0-20160820-SNAPSHOT-ZNH5YAO0J6-oneplus3.zip]. This has to be flashed first.
3. After flashing cyanogenmod, /data/ has to be unmounted in the TWRP mount menu, otherwise the sfos install script will throw an error. (If in doubt, also unmount /cache/, although i had no problems with it)
4. Now sailfishos is ready to flash. Just install the zip and reboot your phone

Note: Something in this process sets the device version to "oneplus3". The original firmware image however expects a capitalized "Oneplus3" and refuses to install. If this happens, just use an archive explorer to remove the first line in the updater-script found at /META-INF/com/google/android. This should be save if you've picked the right zip for your device.

With this steps the install process finished smoothly without errors on my device. This doesn't do it for me though, the boot process doesn't start and im stuck at the white oneplus logo. If anybody could tell me what i did wrong i could change the little guide accordingly.