The charge was one of two that Starbucks has filed against Workers United since the union began organizing late last year. More than 200 Starbucks locations have unionized since December.
The three workers involved were all leaders in union efforts at a Phoenix Starbucks, and claim the company took action against them for their organizing efforts.
Laila Dalton, a shift supervisor who was working to unionize the location at Scottsdale Road and Mayo Boulevard, was fired the day before union election ballots were sent out.