It was a hard work, but I finally found a solution to root Doogee X3 running Android 6.0. Here is how it goes:
1. you need TWRP recovery. Here: http://www.needrom.com/download/twrp-for-android-6-0/
- you will find there also the needed SuperSU. Trust me, I uploaded it there
2. you need SuperSU-v2.61-20151207213702. Download from here if you didn't in the first step: http://www21.zippyshare.com/v/YpqXUZg1/file.html
- put it in root of the external SD card (not sure if it would work if you put this file on the internal root storage)
3. enter TWRP recovery. If you are entering TWRP the first time you'll be asked if you want to enable write permission to system data. LEAVE IT AS READ ONLY ! (not sure whether it matters, but this is how I did)
4. install SuperSU zip file and reboot. If you get a question if you want to root, click NO.
- after phone is restarted, you'll see installed SuperSU in your drawer. If not, download SuperSU by Chainfire manually from the store.
- maybe you want to check root first. get any root checker app from the play store.
- start SuperSU app and update it.
This is the solution how you can root Doogee X3 running Android 6.0. I didn't lost root nor I got a boot stuck after updating SuperSU. So it's flawless. Don't try to install actual SuperSU 2.78 directly. It will not work and it's ending in a boot stuck.
I found out after updating SuperSU via app, that CCleaner shows a false root state. I prefer using Root Check
It will show you if you have busybox installed also. A must have for root users =)
Have fun guys ! I'll hope this will help some guys out there