There's a bit of a tangle here. The three players are:
- savings on name-brand clothes
- a savings of 75%
- 75% off
A. “Get savings of up to 75% off name-brand clothes…”
= “The savings you can get on name-brand clothes are up to 75% off”
Option A simply omits on. Arguably it's kinda awkward: saying the savings are 75% is much nicer than the savings are 75% off, unless you're about to say …off list price or something, but then I'd likely prefer 25% of list to avoid confusion.
B. “Get savings of up to 75% on name-brand clothes…”
= “The savings you can get on name-brand clothes are up to 75%”
This is the option I'd go for. Straight to the point.
C. “Get savings of up to 75% off of name-brand clothes…”
= “The savings you can get off of name-brand clothes are up to 75%”
…No.