Paper with multiple authors
If I understand it correctly, you're a reader of the paper and want to contact the author(s)? 


In most journals, one author is marked the corresponding author (and their email address is provided more prominently. It's a good first port of call, and the right thing to do on recent papers.
Otherwise, depending on the field, the first author is likely to be the principal author and most able to speak to the entire article. 
If the paper is very old of course, you might have to do some research of your own to find out who is still active and which email address is still valid. In this case, if you can't find the first author's (who may have been a junior researcher and moved on) it might be ok to inquire with the lab director. 
If you have all email addresses, you might want to send it to all three. Three is not that many addressees. Five or more would be different - you don't want to send a mass mail.


Also, the content of your letter would be slightly different based on which option you choose. You didn't tell us why you want to email them. (If it is to get a copy, first author is probably the one who is maintaining a personal file.)