The 3800 V-6 came standard in your 2005 Buick LeSabre. The positive crankcase ventilation valve is in the front of the upper intake manifold, hidden under the manifold air pressure sensor.
You'll need to remove the fuel injector shield -- that's the plate on top of the engine. Take off the MAP sensor from the front of the manifold and you'll find the PCV access cover. Use a 16 mm socket to push down and turn the access cover, take the cover off and there's your PCV valve.
