First things first install MTK USB Vcom drivers.
For PC running windows 7 use Adb driver installer. (Google it).
For Windows 8 and above Microsoft has restricted Unsigned drivers installations so first disable driver verification on windows 8- 10.
Go to PC Settings from the Start menu >> Update and Recovery >> Recovery.
Under Advanced Startup, click on Restart Now.
Your PC will do a kind of reboot and go to a blue Choose an option screen. Click on Troubleshoot (refresh or reset your PC or use advanced tools) >> Advanced Options >> Startup Settings (change windows startup behaviour) >> Restart.
Your PC will now restart and go to yet another blue screen with 10 options. Press 7 or F7 on your keyboard to select Disable driver signature enforcement.
Your computer will auto restart again and then you can now proceed with the installation of Mediatek's Preloader USB VCOM driver
Go to Device Manager >> Action >> Add Legacy Hardware
Select Install the hardware that I manually select from a list and click Next >>> Select Show All Devices and hit Next
In the next screen, click on Have Disk in the bottom right corner and then navigate to the drivers you extracted to your computer.
The drivers are located in the Windows 7 folder. For 64 bit PC choose usb2ser_Win764.inf and for 32 bit PC make use of usb2ser_Win7.inf then click OK
Once the driver loads, select MediaTek Preloader USB VCOM Port in the next screen and click on next.
An error in red that says Windows can't verify the publisher of this driver software will pop up. Select Install this driver anyway and the installation of the MediaTek Preloader driver will start.
Now you can proceed to flashing your MTK based Android phone with ease using SP Flash Tool.
After successful installation, I got the error – This device cannot start. (Code 10)
simply ignored them as and run SP Flash Tool
To upgrade to lollipop is quite straight forward. After successfully installing Vcom drivers next download and extract Smart Phone flash tools (latest version) . also download and extract the lollipop Rom .
Open flash_tool.exe a prompt dialog with display alerting you that there is no scatter file, click OK
Select scatter loading file>>> next navigate to the location of the extracted Rom open and then select the Android Scatter file it will be the only file there.
Goto option >>> connection >>> High-speed.
Select Firmware Upgrade from the drop down option and click download.
Now connect your switched off Jiayu S3 via USB cable. Wait for the process to complete you'll see OK
Backup you phone data and maybe even create a Nandroid backup in case you decide to revert back to KitKat ( I wonder who would)
Link to Jiayu S3 5.1 : https://docs.google.com/file/d/0B1ZCUZBU0lhXY01GMlJNZnpuY1E/edit
VCOM .zip - https://www.dropbox.com/s/53zvuti4lo8f72j/VCOM%20drivers.zip?dl=0
Recovery TWRP: https://mega.co.nz/#!2sRUTI5L!4XTPjt7RBEiw8R5aKfiG8HvuiRkT7O8Q29q3kcCxP8U
Gapps : link down, use the one @dljbd1 dropped
Update SU : http://download.chainfire.eu/supersu