Venn Diagrams
This excellent site will give you some practice with set notation involving three sets.
http://math.uww.edu/faculty/mcfarlat/143venn.htm

Venn Diagrams with two Sets
http://www.cns.uni.edu/~campbell/stat/prob2.html

Venn Diagrams with Three Sets
http://www.nist.gov/dads/HTML/venndiagram.html

Multiple Choice Practice with Venn Diagrams
This is a Venn Diagram tutorial.
http://www.engr.iupui.edu/~orr/webpages/cpt120/venn/venn/venintro.htm

Venn World
This is a fun site playing with Venn diagrams.
http://www.themathlab.com/geometry/section2/vennworld.htm