the image is really poor.
That is because composite Video sucks, especially at higher resolutions than the NTSC standard.
The best one can hope for with systems that utilize Composite is that they might also allow for the use of S-Video, which is a tad better than composite video. I use a Composite Video cable that also has a S-Video lead with my old game systems and the results are noticably better.
i have bought vga to 3 rca cable (this has green, blue and red wire)
That is a component video cable.
which has 3 ports one for video and two for audio.
And that is composite video/audio- Yellow for video, white and red for the two audio channels. They are not compatible, in any way, with a Component cable.