Q:

What is a free and simple 3D plot software for students?

I need any plot software on Linux or Windows that my students should use it for plotting 3D functions. 
I want introduce any software that be free and useful for bachelor students.

A:

For undergraduate level, Gnu Octave is probably what you want. It is open source, cross-platform, and syntax-compatible with MATLAB. It's very useful for 2D and 3D plotting and for numeric linear algebra, and it's a tool that will be beneficial for students to know.
Also, GeoGebra is excellent plotting software, and has recently added 3D graphing support. It is not as powerful a tool as Octave, but is well-suited to simple plotting and rendering tasks.

A:

In terms of free software, a large portion of the available choices are based on a Gnuplot backend; I however would probably not recommend directly using Gnuplot. Instead, your choices are mostly between the various front ends. In terms of the front end, depending on what your students are familiar with and are willing to learn, there are many options. 

matplotlib is extremely powerful and configurable, but requires some amount of Python programming from the students. 
Sympy also is based on the basic Python syntax, but its interface and use may be more familiar and accessible to people who have already some familiarity with a computer algebra system. 
maxima and its GUI wxMaxima forms a freely available computer algebra system with a nice interface. In addition to using the gnuplot backend, Maxima also supports a different xMaxima backend for plotting. 
The list goes on, but the gnuplot project page maintains a list of front-ends and programming interfaces using it. 

For 3D plots of algebraic relations (polynomials etc.), a slightly less well-known program is surf, which was intended as a visualization aid for real algebraic geometry. Unfortunately development seems to have stopped several years ago, and recently I have had some difficulty building the source code on my Gentoo Linux machine. Your mileage with it may very well vary. 

A:

You might want to look into Geogebra.  The current beta version (Geogebra 5 Beta) has a 3D mode and allows you to create 3D plots of a function of the form f(x,y) and a graphical interface for rotating, zooming, etc.  It is also open-source, free, and cross-platform.

