Redmi 3S Prime for its price is as good as any phone could be. It has all the features that its flagship brothers have but at a slightly lower price with some compromises as in camera quality, display etc.
In todays scenario this phone is obviously outdated now as we have gone full on bezel-less phones with high-end features which comes at great price but for those who are strictly tight on their budget as in 10K price, this phone might be a good choice.
It has MIUI which is Mis own custom skin of Android with some great customization and is easy to use and user-friendly. It receives regular updates for Android. It has decent camera for its price, HD display(not the best but still you know), IR blaster etc. But the phone has a massive 4000 mAh battery that can get you through whole day. Which in my opinion is not so bad given its price.
I have used this device for around 2-3 years as my primary phone when I couldnt buy those high-end flagship phones but I never felt off even with this phone as it had all necessary features that usually a normal person asks for. And so it was a great phone for me.
So at last, I can just say that this phone my seem obsolete in todays perspective but for those who are just a phone its a good choice at lower price.