How do I install systemless SuperSU in Samsung? I'm currently using a custom Android 6.0 ROM on my Samsung Galaxy Note 3 SM-N900. I need to put suhide in my device and it need systemless SuperSU to be installed, Google didn't help me at all. Any of you guys know how to?
If you are using stock Android 6 and newer you can only have systemless root because of security restriction. So if you root your phone running stock Android 6, you will have systemless root.

Because you are using a Samsung phone it might be a bit more tricky. It could be that your custom rom has it's own su installation in the system partition. Then it's not systemless. To have a systemless root installation it's required to install su in the boot image. This requires your device's bootloader to allow flashing the boot image. 

On most devices you can somehow flash the boot image by unlocking the bootloader.

For more information about what systemless root is have a look here.