#1) 120Hz HDTVs and 120Hz PC monitors operates differently. "120Hz" for HDTVs generally refers to video processing they only accept 60Hz inputs and outputs at 60Hz as well. Video processing basically smooths out video playback and gives video that "live look". The same can be said for 240Hz HDTVs, but they perform even more video processing.
#2) 3D HDTVs should specifically state "3D" somewhere in the specs. To add a little bit of confusion 3D HDTVs are also "120Hz". Therefore, it is important that you look that the specs for "3D supported" or something like that.
You need at least a Samsung 7000 series for "3D". See link below which has a list of Samsung HDTVs. Note where it states "7000 Series 3D 1080p" and underneath it states "3D Enabled". Look at the 6900 Series, no where does it state "3D" or "3D Enabled".