。
She goes to the supermarket every three days.
Literal
She [topic-は] three-days every [target-に] supermarket [to-へ] goes.
ごとに ('every X, at intervals of X') is the standard frequency suffix: 日ごとに ('every day'), 週ごとに ('every week'), 三日ごとに ('every three days'). Slightly more deliberate than the bare 毎 prefix (毎日 'every day,' 毎週 'every week'), and works flexibly with custom intervals. スーパー is the abbreviated form of スーパーマーケット — Japanese loanwords are often clipped to two- or three-mora forms in everyday use: パソコン (personal computer), コンビニ (convenience store), デパート (department store).