I think reason for having 3 brands is so they can have more exclusive distributors in each region. Sort of what Gadini does with Elligi/Power/Fast/Sprint/...etc tires. Except for the Mugen Fast with different rims, all other brand use the same rims and rubbers for touring car tires.
On the external, the engines all have slightly different crank case... The internal porting are exactly the same. If you are after a stock engine I think they are all the same. You have to go to JP/Max/M.Rossi version to get one that has been worked on.