So after years of service, my Mi3 gave out. No blinking lights, screen won't turn on, and download/fastboot mode wouldn't work. Connecting the phone showed an unrecognized USB device. Thought that I would just let it go already, but the fixer in me couldn't help but try.
And amazingly, it's now brought back to life! Here's what I did:
Step 1: Download the flash tool and fastboot rom here http://en.miui.com/a-234.html
Step 2: Install the flash tool along with the drivers in the program
Step 3: Disassemble your device until you have access to the motherboard and battery. You will need a few tools for this. A small hook or a flat screw along with a thin but sturdy card (like a credit card) can pop the back cover open. Here's a good disassembly guide. http://www.myfixguide.com/manual/xiaomi-mi3-disassembly/
Step 4: Remove the plug of the battery from the motherboard. DON'T CONNECT IT YET (Refer to the image attached below)
Step 5: Remove and then connect the USB from the motherboard. The port might not be properly connected. (Refer to the image attached below)
Step 6: Connect your phone to your computer via USB while the battery isn't plugged. Check device manager, click on "ports" and then check if it says "Qualcomm HS -USB QDLoader 9008" If so, you're on the right track.
Step 7: Extract the Fastboot rom you downloaded. Make sure that you've extracted it to the very core. You should see an "image" folder along with a couple of files.
Step 8: Open Miflash with admin rights and then go to the fastboot folder. NOTE: Do not choose the image folder. Choose the previous folder with cancro...
Step 9: Choose "save user data" at the bottom. You can try clean all, although choosing this kept giving me a hello packet error.
Step 10: Click refresh and the COM port should show up which should match the one you saw in device manager. If so start the flash. Mine took about 170 seconds.
Step11: After the flash succeeds, unplug the phone and then reconnect the battery to the motherboard. Then connect the phone to a charger and check if it works. A red blinking light might appear and that's okay. Just leave it plugged for a while and it should start charging normally. Charge it until it's full and then boot up!
I reassembled my phone after I was sure it can boot. It's working pretty well now! Cheers!