this is in short how I installed the sailfishOS port on a oneplus 3. Read this post complete, before you try it by yourself, please. Do it at your own risk and only if you did understand everything.
You should know, howto unlock your oneplus 3, howto install custom ROMs via TWRP and howto use fastboot. It is only possible with unencrypted phones. You can fiind all of it in the referenced xda-developers threads.
You can use SailfishOS with MultiROM only as primary. Every other OS should be able to start as seconday, or you have to use the swap function from MultiROM.
 LineageOS 14.1:
 MultiROM v33(e):
 SailfishOS 18.104.22.168:
 TWRP: ( download: )
I had a TWRP 3.1.2 and the LineageOS 14.1 (with Firmware 19Beta, read all in LineageOS 14.1 refernce) already installed on my oneplus 3. If you want to use SailfishOS and LineageOS with MultiROM on your oneplus 3 you should start with the LineageOS 14.1 installation.
1. Follow  and  to install the official LineageOS 14.1 and TWRP as recovery.
2. Start your LineageOS and make your basic config
3. Follow  to install MultiROM support and the TWRP MultiROM v3.0.3
4. Test your MultiROM installation and start your LineageOS
5. Reboot into recovery and copy your primary LineageOS to secondary
6. Follow  to install SailfishOS 22.214.171.124. You have allready a recovery installed and can use it to wipe System, Data, dalvik art and cache. Right after wiping you can flash the openBeta19 Firmware, lineageOS unofficial 14.1. and sailfishos ZIP files in one shot to primary.
7. Start sailfishOS the first time and make basic config. If you know its UI allready you can skip the tutorial (tap clockwise in the edges, begin at upper left).
8. Reboot and test to boot your (in step 5.) to secondary copied LineageOS