It's probably the motherboard BIOS that has the limitation. Many of the cheaper motherboards are like that. But, there's nothing stopping you from getting a better motherboard and snapping it into your eMachine--and considering that you can keep all the other components excepting maybe the cheapo stock video card, it shouldn't cost you too much to do.
I discovered the BIOS limitation while trying to install dual video cards in an older Compaq system; their BIOS wasn't able to address the two video channels. So, I switched to a newer Dell box, and voila! Dual video. Triple video will be much more of a challenge...