You should be worried, they are obsolete cards with obsolete amounts of VRAM for today's games. Do not buy these cards, waste of money.
Even if three of them deliver a decent amount of performance, you will be bottlenecked by the VRAM in every single game today. Forcing you to seriously lower the graphical settings in games, making it pointless to have 3 of them in SLI since you wont be able to use a single ounce of the power to begin with.
Not at all. SLI 670's perform similar to a single 780 Ti. A single 580 is about 25% slower than a 670.