I cannot recall an interracial threesome where the two women were of different races. I am sure they are out there, but does not seem common. I have seen interracial lesbian scenes, but again kind of rare.  
The one exception is with Asian performers. Then it is always interracial unless the porn is made out of Asia. I think there is one active Male Asian pornstar in America.  

One thing I always find weird is when people ask when Asian American adult actresses are going to do an interracial scene. Every scene they do is an interracial scene. Why don't people just come out and say, "When she is going to get some black cock? "

It's plenty of movies that show that too.