OK, i got my answer, the 3x 780 smoke the the 2x Titan black in Cycles, cause the additional ca. 500 Shader cores are beaten by the additional MHz in speed. So you easily could say that 2 780 are 20% slower than 2 Titan, add another 780 card and you are 50% faster. At 1000 Bucks less!
Ok, saw it tested and can confirm.
Gone are the days of the 6GB VRAM Titan exclusivity for large scenes.