Try this one with two separated lines as below.

1. Define your sets as below.

   var setA = dbContext.SetA; // Represents your A set var setB = dbContext.SetB; // Represents your B set var setC = dbContext.SetC; // Represents your C set

2. Firstly get B except C result.

   var bExceptC = setB.Except(setC);

3. Finally, get A except bExceptC result.

   var result = setA.Except(bExceptC);