Connecting my tv and computer

What do you mean by 3 RCA? Does it have the red, green, blue (component video) or does it have yellow, red, white or black (composite video)? If your computer does not have the proper video card then you will need an adapter. But make sure you know what kind of connection because you can not use one from the other.

i have a red a white and a yellow 3 rca tv

Does your computer indicated it output composite video? Have you change the display setting to extended display or duplicate?