I only listed three references to cover my last two assignments (6 years). SWA will also contact the people who write your LORs (can be the same as those on the list). They want to talk to people who know you well. Lindsey recommended supervisors, but she said peers were OK. They will call everyone on the list, so you might not want to make extra work for them!
Also, the gouge I got was to show up with about 5-6 LORs (definitely more than three). Still waiting on the 15 June DB, so take it for what it is worth.