So simple...

#include <acknex.h>
#include <default.c>
#include <d3d9.h>

// define a 2D vertex struct
typedef struct VERTEX_FLAT { 
	float x,y,z; 
	float rhw; 
	D3DCOLOR color; 
} VERTEX_FLAT;
#define D3DFVF_FLAT (D3DFVF_XYZRHW | D3DFVF_DIFFUSE)


function main()
{ 
	vec_set(screen_color,vector(1,1,1)); // black nontransparent screen

// define and preset the triangle
   VERTEX_FLAT vf[3];
   vf[0].color = 0xFFFF0000; // the red corner
   vf[1].color = 0xFF0000FF; // the blue corner
   vf[2].color = 0xFF00FF00; // the green corner
   vf[0].rhw = vf[1].rhw = vf[2].rhw = 1.0; // no perspective
   
// define three corner vectors   
   VECTOR v[3];
   vec_set(v[0],vector(100,100,0));
   vec_set(v[1],vector(400,100,0));
   vec_set(v[2],vector(400,400,0));

// define the rotation center
	VECTOR center;
	vec_set(center,vector(250,250,0));
	
	wait(1); //wait until the D3D Device is opened after the first frame

// draw a red/blue/green rotating triangle
	var angle = 0;
	while(1) 
	{
// rotate the corner vectors
		int i;
		for (i=0; i<3; i++)
		{
			VECTOR vTemp;
			vec_diff(vTemp,v[i],center);
			vec_rotate(vTemp,vector(angle,0,0));
			vec_add(vTemp,center);
			vf[i].x = vTemp.x;
			vf[i].y = vTemp.y;
			vf[i].z = 0;
		}
		angle += 10*time_step;
		
// open the scene and get the active D3D device
	   LPDIRECT3DDEVICE9 pd3dDev = (LPDIRECT3DDEVICE9)draw_begin();
	   if (!pd3dDev) return;
	
// set some render and stage states
	   pd3dDev->SetRenderState(D3DRS_ALPHABLENDENABLE,FALSE);
	   pd3dDev->SetTextureStageState(0,D3DTSS_COLORARG2,D3DTA_DIFFUSE);
	   pd3dDev->SetTextureStageState(0,D3DTSS_COLOROP,D3DTOP_SELECTARG2);
	
// now draw the triangle
	   pd3dDev->SetFVF(D3DFVF_FLAT);
	   pd3dDev->DrawPrimitiveUP(D3DPT_TRIANGLEFAN,1,(LPVOID)vf,sizeof(VERTEX_FLAT));
	
		wait(1);
	}
}