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, AND-Split and AND-Join.
Hint: You could use BPMN to visualize your result.