Redmi Note 3 is good at performance(CPU), but sucks at gaming(GPU), plus no SD card slot
Redmi 3s is decent at performance(low performance), but have better gaming performance than redmi note 3(Adreno 505 is a new GPU, might be better than Adreno 405 in SD616 in Redmi 3 pro); also have hybrid slot(sd card support in one of sim slot) and fingerprint sensor.
The best one is neither of them; it’s Redmi Note 3 pro with SD 650(great performance, close to last year flagship cpu performance) and Adreno 510(lower than Adreno 530 in SD 820, but better than Adreno 505 in SD 430) as well as 2 options of RAM & storage(2/16 or 3/32) and fingerprint scanner.