Muddle the mint, cucumber, and orange slice. Add everything else, then shake with ice and strain into a chilled Collins glass filled with ice. Garnish with a mint sprig and an orange slice.
"#3 Cup" refers to the base spirit of the bottled Pimms cocktails. There used to be many: #2 was scotch, #3 was brandy, #4 was rum, #5 was rye whiskey, and #6 was vodka. All that remains in the United States is the gin-based #1 Cup. —Gerry Corcoran, Spring 2009