I recently downloaded 3.0.0a and found several missing imports

import org.lwjgl.input.Keyboard;
import org.lwjgl.input.Mouse;

import org.lwjgl.opengl.Display;
import org.lwjgl.opengl.DisplayMode;
import org.lwjgl.opengl.ARBProgram;

should I import Keyboard and Mouse from net.java.games.input ?
where are the other imports now

They are missing because 3.0 uses GLFW for window management

Have a look at the demo code on github for examples of getting events working (mouse and keyboard are examples of events). The demo also shows you how to get a display up, as does the example in the getting started code on the main website.

https://github.com/LWJGL/lwjgl3/blob/master/src/tests/org/lwjgl/demo/glfw/Events.java

Not sure about the ARBProgram, Spasi can comment more on the missing extensions. I would say this extension is very very old and you can do the same thing with core opengl now

Wow, actually someone using the assembly language from ARB_vertex_program and ARB_fragment_program.
But anyhow, the ARBProgram class in LWJGL 2 seems to be an artificial "common superclass" of both ARBVertexProgram and ARBFragmentProgram.
You can use the latter two in LWJGL 3.