How To Install Android Marshmallow Developer Preview 3 in Nexus5|Nexus6|Nexus9

Google has finally declared its next android iteration are going to be named as the sweet treat "Marshmallow". So, Android 6.0 is for "Marshmallow:.
Marshmallow was one among the extremely speculated name that matches Google’s terminology of sweet treats like --- Cup Cake, Eclair, Froyo, gingerbread, Honeycomb, ice cream Sandwich, Jelly Bean, KitKat and Lollipop. It beat different probable names like mud pie, mousse, and our terribly favorite Malai Barfi.
The company discovered the name on its Android developers blogspot and  conjointly released the ultimate Android M 6.0 SDK which will be accessible via the SDK Manager in android Studio. it'll bring access to new Apis and latest build tools.

Marshmallow brings new Features :

Google now on Tap - Users currently don’t have to be compelled to leave an app they require to go looking for anything. For instance: if a person messages you about restaurant or new eating place, you'll be able to then simply raise Google now for details, while not going away from the app you're on.
App Permission - provide users the power to decide on what functionalities their apps can have access to. for example, if a user does'nt want for an app to access his location, he has the power to put off permissions to access location.
Track Memory feature will be handy for users ,let users check which app is consuming proportion of ram on its Phone/Tab.
Quick Finger Print Access won't solely let users unlock their device, however additionally log in to the Google Play Store & to authorize purchases created within the play store.Features like 
Auto Backup can automatically create a backup of the app & its data and put it aside  on Google Drive so users don't lose the data once swapping phones.
Doze Mode on android M will aid in saving battery life because it mechanically activates once the device has not been used for a short while.
Visual Voicemail
And some other Features are :

Rotating home screen
Android Pay 
Intuitive Web Experience
New app drawer
Remove status bar icons
RAM Manager
Network Settings Reset option
Supports USB Type-C


Installing Android M Developer Preview 3

For Windows

Download And Install ADB & Fastboot (While Installing Run it as Adminstrator)

Installing ADB on Linux,Mac.

To install ADB on linux/mac machines is very easy just type the following code in terminal

sudo bash <(curl -s https://raw.githubusercontent.com/corbindavenport/nexus-tools/master/install.sh)

To uninstall ADB

sudo bash <(curl -s https://raw.githubusercontent.com/corbindavenport/nexus-tools/master/uninstall.sh)      
After Installing ADB now enable USB Debugging first.

Note: First take backup of your device data and then proceed further

Follow the below step to flash Android 6.0 in smartphone

1. Download the System Image for your device


2. Then Unlock the bootloader of your device

Note:  you cannot install the Android M without unlocking bootloader

3. Unzip the downloaded files in a folder

4. Open a Terminal and change current directory to  adb folder via command

cd c:\directory name
5.Then type the command below then hit enter.

adb reboot bootloader
6.Now android device turn into fastboot mode.In fastboot mode type:

fastboot oem unlock 
Allow them to erase the data of smartphone.Now,Open the unzipped folder containing the system image which you downloaded earlier.

Now open the terminal and run the below command to install android 6.0

flash-all.bat
This process will take sometime and device may restart in process so be patients