Hehe - I did end up reading all 3, and enjoyed them all, but I'd say I got 90% of the value from volume 1. Volume 2 walks through the BSD implementation of TCP/IP, which is fascinating, but way more detail than you'd ever need to know, and volume 3 goes off into some esoteric topics that seemed promising at the time but mostly ended up being abandoned (along with a brief discussion of HTTP as it was around the 90's).

If you're going to read it, though, find a used copy of the original Stevens' first edition, not that terrible desecrated second edition.