most people will advise against 4k for gaming, as they are still a new enough technology where the response time and input lag, not to mention the hardware required for it is just not worth it. but since you seem to have the money for it, instead of 3 290x's i would suggest 2 780ti's, BUT i only suggest this due to the diminishing returns of 3 cards.
The 780ti is also a faster card and holds up better in multi card configurations in terms of frame-time-variance, this is simply a nvidia thing, but remember, the card is also more expensive.
Almost no one will ever actually recommend 3 gpus, as especially with the 290x, you need some serious cooling in your case.
if you are willing to pay for 3 290x, that is my recommendation for the 2 nvidia cards
If you only want to pay for 2, 290x's then i'd say stick with that option