They're selling three different items, one at a time, over a three week period to celebrate their 10th anniversary.
ーThey're selling three different items, one at a time, over a three week period to celebrate their 10th anniversary.
「彼らは１０周年を記念して、３つの異なるグッズを一種類ずつ三週間に渡って販売する。」
three different items で「３つの異なるアイテム・グッズ」
one at a time で「一つずつ」
over a three week period で「三週間にわたって・及んで」
10th anniversary で「10周年」
ご参考まで！
カナダ 
