A famous paper has three coauthors. I wrote a note regarding that paper and want to get some feedback from them. Shall I send the note and my questions in one email to the three authors together (with two of them in cc line or also in "to" line), or is it more respectful to send three different personalized letters?

If I am sending three personalized letters to each of them, shall I tell them that I also send the letter to the two other authors?

One obvious answer for natural science disciplines is to send one email to the corresponding author only. However, most economic papers have equal coauthors and many economic papers have no corresponding authors or three corresponding authors with email addresses specified. In contrast, the natural science papers usually only have one corresponding.