OP3 buttons mods for the OnePlus Three aka OnePlus 3 is available for download courtesy of XDA developers and this will add the missing open / close menu to the button actions. The developer is also planning to add optional single press behavior of recents or back button in coming releases. Anyway, to use it, you will need to install Xposed module or modded settings apk file.
Right now, you can do the following:
* Add “Open/close menu button action”
* Link to SuperSU in Settings – by using modded apk
* Link to Layers Manager in Settings – by using modded apk
As said before, you will need to have xposed framework and for that, root access is required. Download xposed from here and install it. Reboot for changes to take effect.
If you want to do hacks using modded apk, download it from here. Replace original apk with the modded apk (system/priv-app/Settings) and change permissions to rw-r-r & reboot device.