Are you sure it is Viewsonic and not BenQ? Whichever it is, there ahould be an option in your Monitor’s (On-Screen Display) Menu to select the input. Select the one you want. All monitors these days are intelligent enough to detect the one that is connected and take that. So, connect any one of them. You don’t have to connect all. HDMI is the best option which gives great clarity and sound with a single cable.