Putting 2 flat sides at 90° on a small log.

I want to use some 3" branches in the corners of some raised beds, where one would normally use 2x4s or 4x4s.
Is there an easy way to cut a corner onto a small log?
I only need roughly 12" of it.

maybe try screwing a thin strip of straight, 1/4" thick board, to the log.  then use that as a guide for skill saw or table saw.   table saw or band saw would be easiest.

if you're gonna do a lot of em, I'd build a jig to secure the log and guide the saw blade.  Keep the log long while you're cutting the 90 deg. out so you have something to hold/clamp

a basic sawing jig is just a strip of thin (plywood) screwed to a 2x4.   2x4 on top to guide a skill saw, and screw the log to underside of plywood ...overhanging by the amount you're cutting off.  

for 90 deg, just rotate the log and screw your clean cut to underside of plywood to make 2nd cut

hope this makes sense, probably a picture would work better.

Someone here at Permies recommended this video. I've put it into practice to make two parallel cuts at different ends of a log. But he explains how to layout 90 degree cuts as well.