Asking questions/sending note to the three authors of a paper in economics, three personalized letters?

My solution would be to send one mail. Personalization seems a wasted effort as you don't know the people, I assume. It even seems a bit presumptuous.

But put all three authors on the TO: line, not some there and some on the CC:, line. The latter feels like you are picking a lead author and they may not agree with that.

With twenty authors, the corresponding author solution would be fine, but with only a few, send it to all.

Pick one author - probably the corresponding author.

If I received an email addressed to multiple authors of one of my papers - especially if it required any thinking - then it would go right to the bottom of my todo list, and probably stay there for months.

If it were clearly aimed at me (eg to me, cc’d to the others, and I’m the corresponding author), it’d get a much higher priority.

So - make it clear who you want a response from, else it’ll be everyone’s job - and they’ll all ignore it as a lower priority task.

Whatever you do, please do not send three independent emails. The reason is that if you do, then each of the three may spend time writing similar answers to your question, unaware that the others are doing the same. This wastes their time.

Either pick one corresponding author to email directly (as rhialto suggests), or send a single message addressed to all three (as Buffy suggests). In the latter case, each will at least know that the others have received the same message, and whoever replies first can easily cc the others, letting them know they no longer need to respond unless they want to.