$600 for the reference 4GB models only. The 8GB models necessary for 4K will be considerably more expensive with 3x the power usage of a single card.
All the same, with Nvidia scrapping its support for SLI beyond two cards, the rest of the PC gaming industry is bound to follow suit. With an 80% market share, Nvidia sets the standard for future game development. No point for developers to waste time on 3-way GPU setups when only a fraction of a fraction of users will be affected.
So the drawbacks are: heat, noise, power consumption, game incompatibility, and the loss of the Nvidia exclusives (PhysX, Adaptive VSync, FXAA, MFAA, HBAO+, Ansel, etc.).