The three-blade prop may be an option under an approved STC. The Piper PA-32 Cherokee Six was offered with two engine choices, a 260 Hp Lycoming O-540-E as the Cherokee SIX-260 or a 300 Hp Lycoming IO-540-K as the Cherokee SIX-300. Both have constant-speed props of two blades as standard.
You should have a CFI check you out to fly any aircraft with constant-speed props and the Sixes are substantially larger, heavier and faster than the PA-28-140, 150, 160 or 235 models. The -235 also has a constant speed prop of those Cherokees mentioned., as does the PA-28R- ARROWS.