Grargar: Obviously, they won't include it here, just like they didn't include the Valve characters for the release of Super Time Force Ultra here.
Matruchus: Ok, looking at the games its obvious Zombie Army 1 and 2 are old versions of the game while Zombie Army Trilogy is a remastered version of 1 and 2 which also includes the new Zombie Army 3 content + Valve content. There won't be a Zombie Army 3 standalone version. This is another case like Metro 2033 and Metro 2033 Redux version.
Yeah. They changed the gameplay pretty much with that move. Also the graphics and atmosphere. For example there are not as many different zombie models and fog/lightning have been changed in some places. Some enemies got nerfed hard, the shooting feels laggy/off and a very powerful kick got added. That's the major changes I can think off.
The third part have much more close quarter maps which makes it feel more like a overall third person action game instead of a sniper one.
Many of the changes can probably be attributed to that it also got released on consoles so PC users got a consoled game.
Contrary to all this it's still a fun game and I'm thinking about buying the trilogy on PS4 because I believe it is a much better platform for it.