mixon's clearly the best rb in the backfield, but the question is when it'll be his job. I wouldn't want to start mixon week 1 since I believe he's going to have to "earn" the job so to speak
as for the other two options, I don't really like either but you may as well consult this
http://www.borischen.co/2017/09/2017-fantasy-football-tier-rankings.html
looking at the list, cj anderson is higher than fat rob
edit: at the end of the day though, the running back you choose will be the wrong one as I learned last year, lol