That's the same thing. Graphics are just what is displayed ON your monitor. If you want to game on two or three displays then you need ONE graphics card that not only has the same number of outputs, but is capable of driving that many pixels at the resolution and settings you wish to run them at.
RX 470 is budget card, and that particular one is a budget model even for the 470 series. Most of them HAVE multiple outputs. Even so, unless you are running at a very low resolution, like 720p, with pretty low in game settings, you'll never have playable frame rates trying to game on three or maybe even two monitors. You need a more powerful card unless you are playing very low demand games.