You find the Workflow Patterns at http://workflowpatterns.com
a) Represent a Multi-Choice
(OR-Split) by only using the patterns Exclusive Choice
(XOR-Split), Simple Merge
(XOR-Join) and Parallel Split
(AND-Split). The Multi-Choice
should have three succeeding activities.
b) Represent a 2-out-of-3 Join
by only using the patterns Discriminator
Hint: You could use BPMN to visualize your result.
More information on the Workflow Patterns can be found in the book in chapter 4.1.