Based on what benchmarking sites are showing, the 1080 Ti is both cheaper and faster at rasterizing than the 2080 except in a few cases. Once you turn on the ray tracing in the 2080, the consensus right now is performance will plummet (but there aren't any titles that folks can really benchmark yet,) making any sort of triple monitor setup for gaming out of the question in combination with ray tracing.
Dice has already scaled back the ray tracing in Battlefield V for performance reasons. Betting on good performance with ray tracing right now is not a very safe bet.
Without ray tracing, however, you have to ask yourself if the 2080 is worth it, as it generally lands close to but behind the cheaper 1080 Ti.
Only time, benchmarks, and finally your own decision can say whether paying extra for the potential of ray tracing, and the new AA algorithm NVIDIA is offering is worth the extra money.
As far as a 2080 having any issues with regular gaming using a triple monitor setup, it won't have any more issues than any other NVIDIA card doing the same, so I don't see that as an issue to worry about when considering the 2080. If however you're considering ray tracing across 3 screens, that's a pretty easy no, especially when the card is expected to struggle to ray trace even a single screen while maintaining high detail or high frame rates.