I don't think it's practical as having 2 parents seems to work just fine and the organisms would instinctually know how to breed in the right order. For example, when the first male or however you classified it was fertilized, it could produce a pheremone or undergo a physical change (like hair loss or something) to indicate to the thrid partener that it's ready to breed.
Or this could give a whole new meaning to the term "threesome".
Also, there is also a greater chance for dysjunction to occur when the cells must undergo the entire process of meiosis twice, making this a less effective way of transfering genes IMO.
Cool though...
Makes me wonder, what conditions would cause a species to need to evolve into three different sexes?