Is this triple-monitor for gaming? Or for desktop usage? I wouldn't think that it would be any problem for desktop usage, probably not even with 3x 4K UHD screens. (Although, might not be able to decode 3x 4K60P videos on all three outputs without dropping frames.)
For gaming, I wouldn't be so sure. The GTX 1660 ti is mostly a 1080P gaming card. Running 3x 1080P screens for Gaming, at reasonable settings, might be asking a bit too much from it.
I would go with an RTX 2070 at a minimum for 3x 1080P Gaming screens. Maybe higher.
Another thing to consider, if you just want a wider FOV, is going with an "Ultra--Wide" 1080P or 1440P LCD screen, LG makes a bunch of them. Those would work with a GTX 1660 ti, I think. (*With somewhat lower performance for the 1440P UW option.)
Edit: Remember, 3x 1080P gaming, isn't far off from the requirements for 4K UHD gaming, which is actually 4x 1080P resolution. So, the GTX 1660ti is not really considered a "4K Gaming" card. I would look at ones that are considered "4K Gaming" cards, if you wanted to run 3x 1080P screens. But keep in mind the Bezel issues too. That's one reason that I suggested the Ultra-Wide for gaming.
If you're a stock trader, or something, then yeah, a GTX 1660 ti should be find for 3x 1080P desktop screens though, with the occasional video-playback thrown in.