I had the same question. I had the Redmi 5 Plus 4/64GB, and wanted a better camera with a good battery, and I knew the Redmi Note 5 had a better battery
So I thought:
-Redmi Note 5 pros: Better battery, 3,5mm jack, radio FM
-Mi Note 3 pros: smaller and lighter, NFC, USB C, maybe better camera(???)
Finally I found a Mi Note 3 for 200€ on ebay and I am quite happy with it. I miss FM radio and maybe 3.5mm jack, but battery is great and so is the camera.