If I am correct 3rivers only supplies a steel fitting and a brass fitting nowadays, they used to supply a steel fitting and two half brass fittings.
I would definetly go for the 2 piece setup. Less (moving) parts, easier to use and less work. If you use 2 brass sleeves and 1 steel one you have to fit the softer brass on both pieces, if you have to tap them in places with say a small hamer and a piece of wood there is a chance it could deform.
I have little experience with take downs and would wait for someone else to chime in. Just my opinion....