Lenovo Yoga Tab 3 10" (YT3-X50) factory image, OTA update or backup needed
Since I'm a Chinese, I could browse Chinese sites (including Lenovo's own) and grab what I can get. Here's something I dug up with a wee bit of search.


Factory images (page in Chinese). Available for both F (WiFi) and M (cellular) variants. Click any link with "链接跳转" before them to go to the download landing page, then click what's behind "下载地址" for the actual download site (Baidu Cloud). It does seem to require Baidu Cloud client to be installed, so if you can't download it yourself, give me a heads-up and I'll see if I can host it on my own AFH. The extraction password (if there's any) is "ask.lenovomobile.com".
Flashing tutorial (page in Chinese). Requires an SD card with >4GB free space, and a >50% charged battery. Rename the extracted firmware package (something like YT3-X50F_USR_S000015_1509171725_Q1241_PRC_CN_otafull_factory_update.zip) to * factory_update.zip (as the page says, "*" stands for any amount of characters - I don't know what that means exactly so I'll just leave the translation here). Insert the card into the device when it's powered off, then hold POWER and VOL+ for ~3 secs until it vibrates and boots. The devices should automatically do a factory reset and update from the zip, then reboot subsequently.
Also refer to this starter thread for general info.