>> one that turns out to be the old-generation of the windows logo
Well, that is another modifier key, called super in nix type OSes. Quite handy, in fact, if you need to use a lot of keyboard shortcuts.
>> one that engages the right click on the mouse (why?)
That brings up a context menu, making keyboard-only navigation in many apps quite fast and convenient (eg. text processor, file manager
>> and one called Alt-Gr
That is a very useful key, that allows printing extra characters on standard 101 key keyboards. Quite a handful of EU languages use those on daily basis, not to mention people who know the difference between m-, n- dashes, minus-dash and are not afraid of using other typographic symbols.
But I guess you are one of those kids that think that keyboard could use less keys. I mean who uses all the function keys anyway, right? Then all the other keys, just leave the alphanumeric and you should be set.