Best used iphone for 350 euros (or dollars)?

I would suggest reading: The All Too Common SAD Reality of Buying a Used iPad/iPhone – Apple Community . You want to be careful not to accidentally buy an Activation-Locked and possibly-stolen "brick". There are also several other possibilities – like a used phone being blacklisted, carrier-locked, or permanently banned from a social media service that you want to use – that could ruin your day.

As far as those particular phones go

• All support iOS 18.
• None support Apple Intelligence.
• While the iPhone 12 Pro has an older processor, it has more RAM: 6 GB, versus 4 GB each for the other two.
• They originally shipped with iOS 14.1 (iPhone 12 Pro), iOS 15.0 (iPhone 13), or iOS 15.4 (third-generation iPhone SE). Nobody can say for certain how many future major iOS updates each will get. Still, looking back at the 7, 8, and X, each of those got 5 major updates and then hit the end of the line. If these iPhones get the same number of updates, that would suggest that iOS 19 or 20 might turn out to be the end of the line for them.

For more detailed feature comparisons of these phones, see:

Apple – Compare iPhone models

Please read this before you seriously consider buying a used iPhone.

The All Too Common SAD Reality of Buying … - Apple Community