The MixBot is performing a few tasks for you. First of all it will search the database for the most recent similar race. A similar race is one that was run at the same track, surface, distance and class as the one you are handicapping.
When you see the same horses showing up in the top spots for all three mix combinations you can be pretty confident in the selection.
Sometimes the most recent race will be the same as the most recent favorite or longshot race - if that happens then you will only see two lines of rankings in the MixBot section.