3 Rail Scale is the latest move from 3 rail toy size trains to full O scale 1:48. Most modelers who do this use 3 rail track, but obtain equipment that is full scale. That way they can still use the trucks on the 3 rail cars and the proprietary control systems the 3 rail mfg produce which are quite common.
I have no idea what "moden lionel" but the terms are also used by the Trainz auction site. I think they use it to describe stuff that was made maybe from 1990s or later.
If you want to do scale models, you need to pay attention to how the 3 rail mfg describe their product. They all make a variety of products to various price points. The cheaper stuff is generally not scale. the most expensive is. And there is lots in between. It is best to know what the prototype for the model is, so you can decide if the model is correct and to scale.
As for metal, as mentioned above there is lots of high end brass on the 2 rail market. These are generally short run hand and jig assembled and are models of specific prototypes. the newer stuff made in the last 20-30 years is generally very well detailed to match the prototype. the older stuff going back the 50s and 60s Max Grey and USHobbies is more generic in many cases and not as well detailed. If you want to use this stuff, you would most likely have to replace the wheelsets (or the trucks) so that the wheel flanges are more compatible with the coarser standards used for the 3 rail track.
the "modern" Plastic cars are quite well detailed in many cases. many are based on real prototypes too. Some are better models than brass ones.
Before you jump in and start buying stuff, you might want to decide if you are a collector, or a modeler. A collector by my definition will buy stuff ready made that he likes or fits his plans but might focus on one MFG or Brass or something. A modeler on the other hand will buy and/or build stuff that fits his plan regardless of mfg or what it is made of.
In 2 rail, the National Model Railroad Association has standards and recommended practices that most 2-rail mfg use so the equipment is all compatible.