Sust has multiple esters, some of which have short half-lives. 3x per week is likely the best for Sust to maintain constant blood levels. Sust is not apples to apples with deca which is a single long ester (decanoate) nandrolone. Deca's ester is long enough that you can do once per week injections. Is that optimal? Not really sure, someone should plot it out with the half-life. At the very worst, 2 injects per week is equivalent to one and since the ester's halflife is around 10 days (if I remember right), twice a week might be a bit better for the blood levels.
Of course, more far far more important is your training and diet.