Trying to find a good iPhone

The iPhone SE (1st generation) cannot update past iOS 15. Many popular apps have abandoned support for iOS 15 and the iPhone SE (1st generation). If you are in India, be aware that NPCI does not allow making or receiving UPI payments on anything less than iOS 17.

The iPhone X cannot update past iOS 16. It's a little better off than the iPhone SE (1st generation) when it comes to app availability – but given that iOS 16 is four versions behind iOS 27, you may find developers abandoning iOS 16 before too long.

The iPhone SE (3rd generation) can run the current version of iOS, iOS 27. I believe that this iPhone is the only one of the three that supports connecting to 5G cellular networks.

All of these iPhones are discontinued. That implies that you are thinking of buying a used phone. If that is the case, please read

• The All Too Common SAD Reality of Buying a Used iPad/iPhone – Apple Community
• If you want to buy a preowned iPhone - Apple Support

You do not want to spend your money and get an Activation-Locked "brick", a blacklisted phone, or a phone that is locked to a carrier that you cannot or do not want to use.

SE 3rd Gen is the most advanced of the iPhones that you mention.