a trinagle in  3D

Now I see '3D' in your question - sorry. I think you ask about OpenGL. So it will be something like that:
void GLWidget::paintGL()
{
  glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);
  glLoadIdentity();
  glTranslatef(-1.5f,0.0f,-6.0f);
  glBegin(GL_TRIANGLES);
    glVertex3f( 0.0f, 1.0f, 0.0f);
    glVertex3f(-1.0f,-1.0f, 0.0f);
    glVertex3f( 1.0f,-1.0f, 0.0f);
  glEnd();
}

Here is an example
Hello GL Example
http://doc.trolltech.com/4.5/opengl-hellogl.html

2D Painting Example
http://doc.trolltech.com/4.5/opengl-2dpainting.html

Overpainting Example
http://doc.trolltech.com/4.5/opengl-overpainting.html

OpenGL:
Qt for Embedded Linux and OpenGL
http://doc.trolltech.com/4.5/qt-embedded-opengl.html

Here is a simple tutorial:
http://www.digitalfanatics.org/projects/qt_tutorial/chapter14.html

TUTORIAL: Setting up a basic OpenGL application using Qt
http://episteme.hin.no/wiki/index.php/TUTORIAL:_Setting_up_a_basic_OpenGL_application_using_Qt

Why you don't want to take these two tutorials first?
TUTORIAL: Setting up a basic OpenGL application using Qt
http://episteme.hin.no/wiki/index.php/TUTORIAL:_Setting_up_a_basic_OpenGL_application_using_Qt

OpenGL
http://www.digitalfanatics.org/projects/qt_tutorial/chapter14.html

The last one has the a source code for download ("here" in the end of the page).

Class GLWidget is from the first tutorial. In the end there is a link to download the source code.

class GLWidget : public QGLWidget, public GMWindow {
public:
  GLWidget( QWidget *parent = 0 );
  ~GLWidget();

  void          clear();
  void          initCameras();

protected:
  // ********************
  // Derived QT functions
  void          glDraw();
  void	        initializeGL();
  void          paintEvent( QPaintEvent* e );
  void          paintGL();
  void          resizeGL( int w, int h );
  void	        timerEvent( QTimerEvent *qte );

  // *********************************
  // Derived GMlib::GMWindow functions
  void	        swapBuffers();
  bool	        toggleRun();

private:
  int           _timerID;
};

Select all Open in new window

GLWidget.cpp
GLWidget.h
Application.h
Application.cpp
main.cpp

In the previous comment I took the source code from
TUTORIAL: Setting up a basic OpenGL application using Qt
http://episteme.hin.no/wiki/index.php/TUTORIAL:_Setting_up_a_basic_OpenGL_application_using_Qt

It uses GMLib.

If you don't want this library, take another tutorial and source code from there:
OpenGL
http://www.digitalfanatics.org/projects/qt_tutorial/chapter14.html

OpenGL tutorials are here:
http://nehe.gamedev.net/


nehewidget.h
nehewidget.cpp
ch11.h
main.cpp

You must enable the OpenGL in your .pro file.

QT += opengl

With that directive, the correct include path to Qt+OpenGl headers are appended to your makefile. Then, include  should work.
But again, see the tutorials. Since you are working with GMLib, that already support QT+OpenGL, maybe the GLWidget is already included or a GMWindow is already derived from a QGLWidget, so maybe you are wrong with a multiple inheritance.