The main reason why you would observe different fire types occuring to the same cell in the same timestep is if those fire types are in different primary transition groups. These primary transition groups define the set of transitions that will be simulated as a unit within the model. Transitions belonging to the same primary group can be part of the same transition event and can’t occur more than once to an individual cell in a timestep. My guess is that in your project definitions for transitions you have currently either assigned a different primary group for the three different kinds of fire, or you have not defined primary groups at all, in which case all transition groups are considered primary. If it is important for you that the different fire types belong to different primary groups, you could prevent multiple occurences in one timestep by setting a minimum TST since fire for each of these fire types. Finally, if multiple fires occur to a pixel within a timestep, all types will be recorded in the cummulative area burned.
Let me know if this answers your questions.