Aliexpress
Here are coupons that you can use.
Latest price update (UTC time):
Banggood
Banggood warehouse is located in China.
Link: Banggood product page
Coupon (557.99 USD): od7off
Coupon (563.99 USD): affiliate6
(CouponsFromChina.com tip: put item in your cart and apply coupon code there after clicking “Proceed to checkout”. Click to copy code and open shop)
Exclusive Allowance Coupons: https://bit.ly/3xWgD9k
Old coupons (from the oldest to the newest): BGRM38, aff7off, bgssaff9, BGKtechredmagicbr
(there’s a little possibility that old coupon will work)
Price history and price alert
Here is price history (for all/most countries) for ZTE Nubia Red Magic 3 8/128GB.
Price records source(s): Gearbest, Banggood
Click on image to enlarge. 
You can set price alert (for Banggood).