You can download official Lenovo A3900 stock ROM from the link in this post. It is based on Android 4.4.3 KitKat.
After downloading the firmware from below link, check end of post for flashing instructions.
Also, do not forget to report broken links.
File Name: Lenovo_A3900_4.4.3_20160914.zip
Size: 448 MB
Firmware Link – Download
How to flash
- Download firmware file from above link
- Copy to your phone’s internal storage and reboot to recovery mode
- From recovery screen, select Install update from sdcard
- Choose the file ‘Lenovo_A3900_4.4.3_20160914.zip’
- Confirm that the file is correct and proceed with flashing.
- Wait for update process to complete, then reboot to system.