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.
Lenovo A3900 ROM – Download – Mirror
How to flash
Follow the instructions available via https://www.leakite.com/2016/03/easy-mtk-firmware-flashing-installation.html to learn how to flash.