Will the 2017 15" MBP touchbar work with a Dell U3417W?
Why wouldn't it work?  It's just a monitor and the MacBook Pro is an Intel based computer.  

The MacBook Pro will support 


Up to two displays with 5120-by-2880 resolution at 60Hz at over a billion colors
Up to four displays with 4096-by-2304 resolution at 60Hz at millions of colors
Up to four displays with 3840-by-2160 resolution at 60Hz at over a billion colors


The Dell will support a maximum resolution of 3440 x 1440 at 60 Hz

Just go from DisplayPort to DisplayPort with the proper USB-C to DisplayPort cable (I prefer to avoid conversion to HDMI) and it should work just fine.

For the record, the only monitors I use are Dells.  I have sizes ranging from very old 19" DVI models to 30" Dual Link DVI models and everything in between.  I use a 24" UltraSharp at my remote office with both a MacBook Pro and a Surface Pro and it performs beautifully with zero issues.