computer shuts off after trying to load 3 instances of game

I dont understand why this is happening my computer is set to performance, my specs are
Processor : Intel(r) core(tm) i7-2600k CPU @ 3.40GHz
RAM: 6 GB
64 bit
with 2 450 gs nvidia cards...

Are these specs not good enough? I was running two just fine but whenever the third one is loading, my computer shuts off, this happened 4 times in a row...

[lax]

You have either a hardware or driver issue on your hands. Your specs are fine. Exceeding specs wont cause a shutdown, just extra slowness.

If it doesn't happen instantly, it could easily be a heat issue. Heat issues are incredibly common over time due to failing fans.

It could also be a power supply issue, as you will be drawing more power with the additional instances, particularly on behalf of your 2 video cards. I would consider removing one of the two cards entirely and see if anything changes. 2 cards can be beneficial for multiboxing if used correctly (e.g. SLI is usually not going to help) but for the most part 1 good card is superior to 2.

It could also potentially be drivers for your hard drive or the chipset that runs it. This is far less common, but throwing this out there because it's easily overlooked.