Spoonerisms are usually thought of as involving the switch of initial sounds in a pair of words, as in prairie schooner/scary pruner. But as Susan Thorpe points out in her article "Spooneristic Variations" in the May 2007 Word Ways, there is no reason why a spoonerism cannot involve three or more words.
Word Ways: Vol. 40
, Article 12.
Available at: http://digitalcommons.butler.edu/wordways/vol40/iss3/12