[museav]

You cannot directly convert VGA and video, you have to either scale video up to VGA or scan convert VGA down to video. Quite frankly, unless you spend a lot on very high quality products, scan converting computer down to composite video, distributing it to multiple projectors and then displaying it on large screens will probably always be of limited quality and there will be some limits even with the best products.

What you probably really need is a seamless switcher and a VGA distribution amplifier, that way you can cleanly switch between sources, obtain an output signal that matches the native resolution of the projectors and then distribute it to all three projectors. However, I don't know if this is an approach that you can budgetarily consider.