The first 2 are DLLs that allow you to alter things in mIRC and the last just lets you create dialogs more easily (at least some people like using it... I prefer manually making them).
Look on google or any other search engine for "DCX DLL" or "nicklust DLL" and you'll find information about each. I believe nicklust is specific to changing how the nick list looks (you can add images, change colors, etc). DCX adds more control to dialogs by allowing changing fonts (color, size, font type), adding rich edit boxes to the dialogs, etc.