If you have a Lenovo K3 Note then Download Lenovo K3 Note Manager v3.1.You can install latest TWRP recovery and SuperSu on your device to get root access.It is a very simple and lightweight tool.It works like LGUP and Odin flash tool. As you guys know Lenovo K3 Note is now updated to latest Android 6.0 Marshmallow.The latest Android marshmallow 6.0 is now supported.You can easily root your Lenovo K3 Note running the latest official Marshmallow 6.0 using  Lenovo K3 Note Manager v3.1.According to the developer, this tool supports Rooting of Build number K50a40_S121_150813_ROW →To know more about Lenovo K3 Note Manager v3.1, please visit the development page. Another method:

How To Root Lenovo K3 Note On Marshmallow 6.0

Downloads

Download Lenovo K3 Note Manager 3.1 (Latest) Download Drivers for Lenovo K3 Note

If you want to know about the features of the tool and how to root k3 note using the tool please visit the Xda thread. Source Xda

Download Lenovo K3 Note Manager v3 1  All In One Tool  - 75