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.