In Harper Lee's To Kill a Mockingbird, how might Maycomb and the events of the novel be different if there were more than two races represented in...

A lot of this answer depends on what the race relations are like between the three groups.  Do they all get along and treat each other fairly?  If so, then there hopefully wouldn't be any "us" vs. "them" mentality during the court battle.  That's naive of me to think though.  

Another unknown is what are the population percentages of the races?  Is it an even 33% per race?  Or is it 50% white, 25% black, and 25% third race?  If it's the latter, I don't think anything changes in the book.  Likely there would still be the us vs. them mentality from the minorities toward the majority, but the total split would be 50/50.  However, if the split was in thirds, then the "non-white" races have a majority. I doubt the lynch mob scene happens then.  I also see something as drastic as Tom being found innocent as a possibility in order to keep the peace.  Even if he were to be found guilty, I don't think he would have been gunned down in quite the same way.  More races in the novel would likely make the people of Maycomb more tolerant of other races.