Answered
I really hate to bother the people here if I don't have to, but I am finding that the manual usually can not answer my questions. For example, I have a problem with a function of 3 variables, and in the algebra window it is titled as a "Multivariable Function". When I search the manual for this I get no hits, but a request to create a new page about this topic!
It would seem that "Multivariable Function" is a well-defined term in Geogebra, but not according the manual. I just want to know how to learn how to use this wonderful tool, but am very frustrated in trying to do so!
The same question 
I guess that there is no page about Multivariable functions, because this term is too general :)
Input is the same as for 2D functions, so you can basically use functional notation and assign a name to the function, or type in directly its expression, and GeoGebra automatically will name it for you.
Commands related to 3D "things" are e.g. Surface(), that uses an intuitive syntax. If one knows how to parametrize (does this word exist? hope so) a function, they can use this command.
Pressing F1 after selecting a tool or entering a command automatically opens a quick help or offers the possibility to open the related manual page, where you can read possible limitations on the use of a command in 3D. In fact some commands can currently be applied only on planes and quadrics, not on any 3D object.
Anyway (sorry for being so long-winded) if in doubt, just ask ;)
Comments have been locked on this page!