Hi Eric, I cant speak from personal experience of actually using it, so see what others say who may have.
As it is considered a soft wood, and recommended as an alternative to Western Red Cedar, I myself would be wary, but like I said I don't know. I always prefer hardwoods for necks. But Redwood is listed as stable and strong so a suitably dimensioned piece with a rosewood fingerboard and only three strings is worth a try. If the grain is on the quarter all the better.
If it was me I would screw or bolt the neck on so I could swap it if not happy later.
I've seen a few guitar builds over on the TDPRI Guitar Forum using Redwood, but it was mostly used on the bodies. I would do a big google search on it and read through the builds first.
It's a real nice looking wood. If it's soft, then a trussrod will probably be a must have item. I think Maple and Oak make better and more stable necks. I've been using Red Oak for necks and fretboards for quite awhile with great results. Most of my 6 string guitars have Maple necks and I love them.
I’ve used redwood on one stringers,but wouldn’t use it on a 3 stringer without a hardwood fretboard.
Another thought is to laminate the neck, by alternating grain directions?
Use it for body’s only as it will bow even with a hardwood fretboard. At least mine did and only a 3 string in openD