One model/table to store data or more?

I am using three different models to store three different INCIDENT data (three different incident typesw with different details). For each of the three models, I can have three models to store INVESTIGATION data, for each of the three, or only one model (Investigation data is similar for all three). I am expecting to have large amount of data gathered in the database. Which is better, three INVESTIGATION models for each INCIDENT or there is not much difference in speed for only one INVESTIGATION model?