I have a HP G70 laptop, I already use the HDMI to plug a monitor in it and it works just fine. I want to plug another one on the VGA port to have 3 screens total. Would this work even though I only have 1 video card?

I can't test it because I haven't bought the monitor yet, I don't want to waste the money

which g70
what kind of outputs
what video card does it have
if no video card, then what processor is in it
what resolutions are the displays and what kind of connectivity does each have

etc

What does it say in the manual?

G70T-200
Mobile Intel(R) 4 Series Express Chipset Family
Core 2 Duo 2GHz
Built-in goes up to 1440x900, 1 monitor goes to 1920 x 1080(It works in that res, I use the HDMI port for this one), the monitor I might buy will be for 1280x1024 or something in that range(Will use the VGA port)

Often if it has two outputs it can support two simultaneous displays, but the built in LCD is normally one of these.

Nope. The vga will mirror. The hdmi will allow dual monitors.

And this is true for every video card?

You cant split the video into 3 screens like im assuming you want . Theres no drivers or cards do that. They stopped supporting vga upgrades about 10 hrs ago.

But you can probably do 2 external with laptop closed