Converting to 1x11 SRAM drivetrain with 8/9/10-speed HG Freehub I currently have a 3x9 Shimano drivetrain, and I'd like to replace the current Shimano 9-speed rear derailleur/shifter/cassette and front derailleur/3 chainrings with the SRAM PG-1170 11-32T cassette, NX rear derailleur, GX shifter, and a narrow wide chainring.

As I understand, 11-speed SRAM cassettes that use an 11 tooth small cog do not require the XD driver body. Additionally, SRAM 11T and Shimano MTB 11-speed cassettes are cross-compatible with Shimano and SRAM derailleurs/shifters. And Shimano 11-speed MTB cassettes fit onto 8/9/10 speed hubs, but Shimano 11-speed road cassettes do not. 

The SRAM PG-1170 cassette is listed as a 'road cassette.' The REI description for the PG-1170 lists 'Shimano HG/SRAM PG' Freehub compatibility.

Is my understanding correct, and would this 1x11 upgrade be as straightforward as replacing the existing Shimano components with the SRAM components described above?
11-speed SRAM cassettes that use an 11 tooth small cog do not require the XD driver body ... Shimano 11-speed MTB cassettes fit onto 8/9/10 speed hubs, but Shimano 11-speed road cassettes do not.


Basically correct. MTB cassettes with large low-gear sprockets can be made with the spider overhanging the drive side of the hub a little, so the longer freehub isn't required. I've been told on this site that some larger 11 speed road cassettes also do this and will fit on a 10 speed freehub.

I'm not sure this is a totally hard a and fast rule, so I think what you'll need to do is ensure that whichever cassette you buy will fit in a 10 speed freehub body 


  SRAM 11T and Shimano MTB 11-speed cassettes are cross-compatible with Shimano and SRAM derailleurs/shifters.


Yes.


  would this 1x11 upgrade be as straightforward as replacing the existing Shimano components with the SRAM components.


The conversion should be straightforward. One difficulty may be finding a narrow-wide chainring that fits properly on your triple crank. Shorter chainring bolts are typically required to attach a single middle ring rather than middle and large.