MLA 8th edition has simplified the rule for citing multiple authors.
Three or more authors are all treat the same way, using the words et al. This is not an abbreviation! See below:
Works Cited List:
Author Surname, First name, et al. Book Title: Subtitle. Publisher, Year.
Example:
Johnson, Norine G., et al. Beyond Appearance: A New Look at Adolescent Girls. American Psychological Association, 2009.
In-Text Citation:
(Author Surname, et al. page number)
Example:
(Johnson, et al.172)