Below is the direct link to download the Lenovo A3900Stock ROM (Firmware).
Note: Follow the instructions on how to flash this device carefully, I will not take responsibility on whatever may happen to your device. Make sure you are downloading the exact ROM of the device you want to flash because flashing a wrong preloaderwill make the device completely dead (can’t power on, can’t charge and will not be detected by a computer).
STEP 1:Download & extract the Stock ROM (Firmware) on your computer. STEP 2: Install the USB Drivers (skip this step if drivers are already installed). STEP 3: Launch SP Flash Tool. STEP 4: Click on Download. STEP 5: Select Scatter-loading. STEP 6: Select the Scatter File from the Stock ROM (Firmware) you have downloaded. STEP 7: Select Format and click Start. STEP 8: Connect device >> holding volume + and volume – button of your device at the same time or holding power button only. STEP 9: Patiently wait for device to be formatted. STEP 10: After format, STEP 11: Select Download and click Start. STEP 12: Connect device & Patiently wait for the process to complete.
[*]It is always advisable to backup the Stock ROM (Firmware) of the device you are about to flash unless the device is completely dead (can’t power on).
==>> Learn how to backup Stock ROMs (Firmware’s) Before flashing from here
[*]Take a Backup of your personal data before flashing or installing any Stock ROM (Firmware) or Flash file on your smartphone. As this helps you to recover the personal data when ever any thing wrong happens.