With a triplex there needs to be a greater consideration in maximising built form verses the common property required to service it. These types of projects start to provide economy of scale benefits when applied to build cost and land cost per site.
A triplex also provides greater commercial flexibility when looking at partial hold strategies and holding cost minimisation. They are also still considered a residential development in the eyes of most lending institutions and thus making funding a little less complex than their larger siblings.
Overall, a successful triplex development can yield greater financial returns both from a revenue standpoint and realisation of profit on sale than more modest sized developments.