3x 1080ti's will b easier to run on a single PSU for cabling, and it is less GPU's to trouble shoot if one of them is overclocked too high and causing crashes after 6 hours. I'm in the minority who prefers to run more powerful cards over quantity and for what its worth more quantity per rig pulls in a greater ROI
For the EVGA G2/P2/T2 line this ends up being an ISSUE as they only have 4 connections for VGA cables, and their "dual" cables are always 8+6 - so the G2/P2 850 definitely prefers the 3 x 1080 ti setup as it has the right cabling out of the box to support that.
The G2/P2 850 also has a shortage on available connections to power risers with for a 5 card rig.
The EVGA G2L line is an odd duck with it's "universal" connectors, I believe a G2L 850 it would work either way - but I got shorted on power cables on the only one I have so I'm not SURE on that.
For Seasonic X-850 or SS-860, not an issue either way - more VGA cable and connections available, AND more "peripheral" connections to power risers.
If you are going to push the cards well past their more efficient point, you have to go to at least a 1000 watt PS and the connections issues no longer matters at all.
IF you set them for the same power usage, the 1070 ti cards will provide more hashrate (on ZEC anyway, haven't played with the 1070 ti at all on anything else but ETH which is a BAD choice) - tradeoff is the "more cards to manage/space taken" question, and it's pretty close either way.
Neither option is a BAD option, it's more about personal preference on "rig density" vs "best hash/$ + hash/watt" and the efficiency isn't majorly different either way.